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{{Short description|Ratio of lengths of fingers}}
{{notability}}
{{distinguish2|[[Benford's law]] about the frequencies of leading digits of numbers}}
{{for|the observation about the frequencies of leading digits of numbers|Benford's law}}
{{Lead too short|date=June 2023}}
[[File:Hand zur Abmessung 2D4D.jpg|thumb|Hand with index finger being shorter than the ring finger, resulting in a small 2D:4D ratio, pointing to a high exposure to testosterone in the uterus.]]
{{Use dmy dates|date=December 2023}}
The '''digit ratio''' is the ratio of the lengths of different [[finger|digit]]s or fingers typically measured from the bottom crease where the finger joins the hand to the tip of the finger.{{Citation needed|reason=source needed on proper methodology.|date=March 2012}} It has been suggested by some scientists that the ratio of two digits in particular, the 2nd ([[index finger]]) and 4th ([[ring finger]]), is affected by exposure to [[androgen]]s e.g. [[testosterone]] while in the [[uterus]] and that this 2D:4D ratio can be considered a crude measure for prenatal androgen exposure, with lower 2D:4D ratios pointing to higher androgen exposure. The 2D:4D ratio is calculated by dividing the length of the index finger of the right hand by the length of the ring finger. A longer index finger will result in a ratio higher than 1, while a longer ring finger will result in a ratio of less than 1.
[[File:Hand zur Abmessung 2D4D.jpg|thumb|Hand with index finger being shorter than the ring finger, resulting in a small 2D:4D ratio.]]
[[File:Hand photograph dorsal1 copy.jpg|thumb|Image shows the measurement procedure of dorsal digit length using [[vernier calipers]]. The hand is placed on the edge of a table so that fingers are rested on table while making an angle of 90 degrees to the palm.]]
The '''digit ratio''' is a pseudoscience involving the ratio taken of the lengths of different [[finger|digit]]s or fingers on a [[hand]].


The most commonly studied digit ratio is that of the 2nd ([[index finger]]) and 4th ([[ring finger]]), also referred to as the 2D:4D ratio, measured on the [[Hand|palm]] side. It is proposed that the 2D:4D ratio indicates the degree to which an individual has been exposed to [[androgens]] during key stages of [[fetal]] development. A lower ratio has been associated with higher androgen exposure, which would be the physiological norm for males but may also occur in some exceptional circumstances in females. The latter include developmental disorders such as [[congenital adrenal hyperplasia]].
The 2D:4D digit ratio is [[sexually dimorphic]]: while the second digit is typically shorter in both females and males, the difference between the lengths of the two digits is greater in males than females.


The 2D:4D ratio has been postulated to correlate with a range of physical and cognitive traits in childhood and adulthood, including personality traits such as assertiveness in women, aggressiveness in men, and cognitive abilities such as numerical skills.<ref name="Jeevanandam_2016">{{cite journal | vauthors = Jeevanandam S, Muthu PK | title = 2D:4D Ratio and its Implications in Medicine. | journal = Journal of Clinical and Diagnostic Research | volume = 10 | issue = 12 | pages = CM01–CM03 | date = 2016 | pmid = 28208851 | doi = 10.7860/JCDR/2016/21952.9000 | s2cid = | pmc = 5296424 }}</ref> It has also been shown to vary considerably between racial groups<ref name="Manning_2000" /> with males having, on average, lower 2D:4D ratio than females.<ref>{{Cite journal |last1=Nayak |first1=Satheesha B. |last2=Nair |first2=Dhiviah |last3=Ravi |first3=Vimal |last4=Aithal |first4=Ashwini P. |date=2018-09-20 |title=A comparative study on digit ratio and hand patterns of three ethnic races of Malaysia |journal=Egyptian Journal of Forensic Sciences |language=en |volume=8 |issue=1 |pages=56 |doi=10.1186/s41935-018-0087-1 |doi-access=free |issn=2090-5939}}</ref>
A number of studies have shown a correlation between the 2D:4D digit ratio and various physical and behavioral traits.


Studies in this field have drawn criticism over questionable statistical significance and difficulties in reproducing their findings<ref name="No relationship between the digit r">{{cite journal | vauthors = Kowal M, Sorokowski P, Żelaźniewicz A, Nowak J, Orzechowski S, Żurek G, Żurek A, Juszkiewicz A, Wojtycka L, Sieniuć W, Poniatowska M, Tarnowska K, Kowalska K, Drabik K, Łukaszek P, Krawczyk K, Stefaniak T, Danek N | display-authors = 6 | title = No relationship between the digit ratios (2D:4D) and salivary testosterone change: Study on men under an acute exercise | journal = Scientific Reports | volume = 10 | issue = 1 | pages = 10068 | date = June 2020 | pmid = 32572079 | pmc = 7308391 | doi = 10.1038/s41598-020-66915-9 | bibcode = 2020NatSR..1010068K }}</ref><ref name="Leslie_2019" /><ref name="Jones_2020" /> as well as lack of high quality research protocols.<ref name="Smoliga_2021" />
==History of digit ratio research==
That a greater proportion of men have shorter index fingers than ring fingers than do women was noted in the scientific literature several times through the late 1800s,<ref name="Ecker A 1875 68–74">{{Cite journal|author=Ecker A |title=Einige Bemerkungen über einen Schwankenden Charakter in den Hand des Menschen[Some remarks about a varying character in the hand of humans] |journal=Archiv fur Anthropologie |volume=8 |pages=68–74 |year=1875}}</ref><ref name="Baker F 1888 51–75">{{Cite journal|author=Baker F |title=Anthropological notes on the human hand |journal=The American Anthropologist |volume=1 |pages=51–75 |year=1888 |doi=10.1525/aa.1888.1.1.02a00040}}</ref> with the statistically significant sex difference in a sample of 201 men and 109 women established by 1930,<ref name="George R 1930 199–204">{{Cite journal|author=George R |title=Human finger types |journal=Anatomical Record |volume=46 |pages=199–204 |year=1930 |doi=10.1002/ar.1090460210|issue=2}}</ref> after which time the sex difference appears to have been largely forgotten or ignored. In 1983 Dr [[Glenn Wilson (psychologist)|Glenn Wilson]] of [[King's College London|King's College, London]] published a study examining the correlation between assertiveness in women and their digit ratio.<ref name=Wilson1983>{{Cite journal|doi=10.1016/0191-8869(83)90061-2 |title=Finger-length as an index of assertiveness in women |year=1983 |last1=Wilson |first1=Glenn D. |authorlink1=Glenn Wilson (psychologist) |journal=Personality and Individual Differences |volume=4 |issue=1 |pages=111–2}}</ref> This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.<ref>{{Cite journal | last1 = Wilson | first1 = G. | year = 2010 | title = Fingers to feminism: The rise of 2D:4D | url = | journal = Quarterly Review | volume = 4 | issue = | pages = 25–32 }}</ref> Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.<ref name=Wilson1983/> In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children<ref>{{Cite journal|author=Manning JT, Scutt D, Wilson J, Lewis-Jones DI |title=The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen |journal=Hum Reprod |volume=13 |pages=3000–3004 |year=1998 |pmid=9853845 |doi=10.1093/humrep/13.11.3000 |issue=11}}</ref> and further developed the idea that the index was a marker of prenatal sex hormones. Since then research on the topic has burgeoned around the world.


==Measurement==
A 2009 study in ''Biology Letters'' argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common [[Allometry|allometric]] line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women,"<ref>{{Cite journal|author=Kratochvíl L, Flegr J |title=Differences in the 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line |journal=Biology Letters |volume=5 |issue= 5|pages=643–6 |year=2009 |month=October |pmid=19553247 |doi=10.1098/rsbl.2009.0346 |pmc=2781964}}</ref> which may be the basis for the sex difference in digit ratios and/or any putative hormonal influence on the ratios.
The 2D:4D ratio is calculated by dividing the length of the [[index finger]] by the length of the [[ring finger]] of the same hand. Other digit ratios are also calculated similarly in the same hand.


The digit length is typically measured on the palmar (ventral, "palm-side") hand, from the midpoint of the bottom crease to the tip of the finger.<ref name="Mayhew_2007">{{cite journal | vauthors = Mayhew TM, Gillam L, McDonald R, Ebling FJ | title = Human 2D (index) and 4D (ring) digit lengths: their variation and relationships during the menstrual cycle | journal = Journal of Anatomy | volume = 211 | issue = 5 | pages = 630–638 | date = November 2007 | pmid = 17764524 | pmc = 2375787 | doi = 10.1111/j.1469-7580.2007.00801.x }}</ref> Measurement of the digits on the dorsal ("back-side") hand, from the tip of the finger to the proximal phalange-bone protrusion (which occurs when digits are bent at 90&nbsp;degrees to the palm) has also recently gained acceptance.<ref name="Kumar_2017">{{cite journal | vauthors = Kumar S, Voracek M, Singh M | title = Sexual Dimorphism in Digit Ratios Derived from Dorsal Digit Length among Adults and Children | journal = Frontiers in Endocrinology | volume = 8 | pages = 41 | date = 6 March 2017 | pmid = 28321205 | pmc = 5337486 | doi = 10.3389/fendo.2017.00041 | doi-access = free }}</ref><ref name="Auger_2011">{{cite journal | vauthors = Auger J, Eustache F | title = Second to fourth digit ratios, male genital development and reproductive health: a clinical study among fertile men and testis cancer patients | journal = International Journal of Andrology | volume = 34 | issue = 4 Pt 2 | pages = e49–e58 | date = August 2011 | pmid = 21091719 | doi = 10.1111/j.1365-2605.2010.01124.x | doi-access = free }}</ref> A study has shown that, compared to the palmar digit ratio, the dorsal digit ratio is a better indicator of bone digit ratio.<ref name="Auger_2011"/> Furthermore, the palmar digit ratio is affected by the differential positioning of flexion creases.<ref name="Kumar_2019">{{cite journal | vauthors = Kumar S, Voracek M, Singh M | title = Differential Placing of Flexion Creases Contributes to Sex Differences in the Second-to-Fourth Digit Ratio (2D:4D) | journal = Frontiers in Endocrinology | volume = 10 | pages = 537 | date = 2 August 2019 | pmid = 31428058 | pmc = 6688535 | doi = 10.3389/fendo.2019.00537 | doi-access = free }}</ref>
A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development."<ref>{{cite journal | author = Zhengui Z., Cohn M. J. | year = 2011 | title = Developmental basis of sexually dimorphic digit ratios | url = | journal = Proceedings Of The National Academy Of Sciences Of The United States Of America | volume = 108 | issue = 39| pages = 16289–16294 | doi = 10.1073/pnas.1108312108 }}</ref><!-- see Simon Baron-Cohen at http://www.edge.org/responses/what-is-your-favorite-deep-elegant-or-beautiful-explanation -->
The formation of the digits, in utero, is thought to occur at 14 weeks, and the bone-to-bone ratio is consistent from this point into an individual’s adulthood.<ref>{{cite journal | author = Garn S. M., Burdi A. R., Babler W. J., Stinson S. | year = 1975 | title = Early prenatal attainment of adult metacarpal-phalangeal rankings and proportions | url = | journal = American Journal of Physical Anthropology | volume = 43 | issue = 3| pages = 327–332 | doi = 10.1002/ajpa.1330430305 }}</ref> During this period if the fetus is exposed to androgens, the exact level of which is thought to be sexually dimorphic, the growth rate of the 4th digit is increased, as can be seen by analyzing the 2D:4D ratio of opposite sex dizygotic twins, where the female twin is exposed to excess androgens from her brother in utero, and thus has a significantly lower 2D:4D ratio.<ref>{{cite journal | doi = 10.1016/j.yhbeh.2005.08.003 | title = Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins | year = 2006 | author=van Anders SM, Vernon PA, Wilbur CJ | journal = Hormones and Behavior | volume = 49 | issue = 3 | pages = 315–9 | pmid = 16143332 }}</ref>


It has been alleged that, because academics have accepted a variety of techniques and equipment (such as [[calipers]], [[radiography]] and [[Photocopier|photocopy]]), researchers record multiple measurements and report only those which have [[P-Hacking|significant findings]],<ref name="Smoliga_2021">{{cite journal | vauthors = Smoliga JM, Fogaca LK, Siplon JS, Goldburt AA, Jakobs F | title = Giving science the finger-is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study | journal = BMJ | volume = 375 | pages = e067849 | date = December 2021 | pmid = 34911738 | pmc = 8672321 | doi = 10.1136/bmj-2021-067849 }}</ref> a form of [[reporting bias]].
Importantly, there has been no correlation between the sex hormone levels of an adult and the individual’s 2D:4D,<ref>{{cite journal | author = Hönekopp Johannes, Bartholdt Luise, Beier Lothar, Liebert Andreas | year = | title = Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: New data and a meta-analytic review | url = | journal = Psychoneuroendocrinology | volume = 32 | issue = 4| pages = 313–321 | doi = 10.1016/j.psyneuen.2007.01.007 }}</ref> which implies that it is strictly the exposure in utero that causes this phenomenon.


==History of research==
A major problem with the research on this topic comes from the contradiction in the literature as to whether the testosterone level in adults can be predicted by the 2D:4D ratio,<ref>{{cite journal | author = Hönekopp Johannes, Bartholdt Luise, Beier Lothar, Liebert Andreas | year = | title = Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: New data and a meta-analytic review | url = | journal = Psychoneuroendocrinology | volume = 32 | issue = 4| pages = 313–321 | doi = 10.1016/j.psyneuen.2007.01.007 }}</ref> but male sexual traits that are stereotypically attributed to testosterone levels have been found in correlation with the 2D:4D. So there should either be a correlation with one or the other but not both.
Various reports in the scientific literature of the late 1800s noted that a greater proportion of men have shorter index fingers than ring fingers as compared to women.<ref name="Ecker A 1875 68–74">{{cite journal |volume=A1|doi=10.1525/aa.1888.1.1.02a00040 |title=Anthropological Notes on the Human Hand |journal=American Anthropologist |pages=51–76 |year=1888 | vauthors = Baker F |doi-access=free }}</ref> By 1930, statistically significant sex differences in digit ratio were established in a sample of 201 men and 109 women, <ref name="Ruggles_1930">{{cite journal |doi=10.1002/ar.1090460210 |title=Human finger types |journal=The Anatomical Record |volume=46 |issue=2 |pages=199–204 |year=1930 | vauthors = Ruggles G |s2cid=83821071 }}</ref> after which time the sex difference appears to have been largely forgotten or ignored.


In 1983, [[Glenn Wilson (psychologist)|Glenn Wilson]] of [[King's College London]] published a study examining the correlation between assertiveness in women and their digit ratio, which found that women with a lower 2D:4D ratio reported greater assertiveness.<ref name="Wilson_1983">{{cite journal |doi=10.1016/0191-8869(83)90061-2 |title=Finger-length as an index of assertiveness in women |journal=Personality and Individual Differences |volume=4 |pages=111–112 |year=1983 | vauthors = Wilson GD }}</ref> This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.<ref>{{Cite journal | vauthors = Wilson G | year = 2010 | title = Fingers to feminism: The rise of 2D:4D | journal = Quarterly Review | volume = 4 | pages = 25–32 }}</ref> Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.<ref name="Wilson_1983"/>
==Digit ratio distribution==
From a study of 136 males and 137 females:<ref name="Bailey & Hurd 2005">{{Cite journal|author=Bailey AA, Hurd PL |title=Finger length ratio (2D:4D) correlates with physical aggression in men but not in women |journal=Biological Psychology |volume=68 |issue=3 |pages=215–22 |year=2005 |month=March |pmid=15620791 |doi=10.1016/j.biopsycho.2004.05.001 |laysummary=http://www.livescience.com/193-finger-length-predicts-aggression-men.html |laysource=[[LiveScience]] |laydate=2 March 2005}}</ref>
* Males: mean 0.947, [[standard deviation]] 0.029.
* Females: mean 0.965, standard deviation 0.026.
Assuming a [[normal distribution]], the 10th and 90th [[percentiles]] for males are 0.910 and 0.984. For females: 0.932 and 0.998.


In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children<ref name="Manning_1998">{{cite journal | vauthors = Manning JT, Scutt D, Wilson J, Lewis-Jones DI | title = The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen | journal = Human Reproduction | volume = 13 | issue = 11 | pages = 3000–3004 | date = November 1998 | pmid = 9853845 | doi = 10.1093/humrep/13.11.3000 | doi-access = free }}</ref> and further developed the idea that the index was a marker of prenatal sex hormones. Since then, research on the topic has burgeoned around the world.
==Evidence of androgen effect on digit ratio==
* Women with [[congenital adrenal hyperplasia]] (CAH), which results in elevated androgen levels before birth, have lower, more masculinized 2D:4D on average.<ref name=Brown2002>{{Cite journal|author=Brown WM, Hines M, Fane BA, Breedlove SM |title=Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia |journal=Hormones and Behavior |volume=42 |issue=4 |pages=380–6 |year=2002 |month=December |pmid=12488105 |doi=10.1006/hbeh.2002.1830 |url=https://www.msu.edu/~breedsm/pdf/CAHFingersFinal.pdf}}</ref><ref name=Okten2002>{{Cite journal|author=Okten A, Kalyoncu M, Yariş N |title=The ratio of second- and fourth-digit lengths and congenital adrenal hyperplasia due to 21-hydroxylase deficiency |journal=Early Human Development |volume=70 |issue=1–2 |pages=47–54 |year=2002 |month=December |pmid=12441204 |doi=10.1016/S0378-3782(02)00073-7}}</ref><ref name=Ciumas2009>{{Cite journal|doi=10.1093/cercor/bhn160 |author=Ciumas C, Lindén Hirschberg A, Savic I. |title=High fetal testosterone and sexually dimorphic cerebral networks in females |url=http://cercor.oxfordjournals.org/content/19/5/1167.full?view=long&pmid=18854582 |journal=Cereb Cortex |volume=19 |issue=5 |pages=1164–72 |year=2009 |pmid=18854582}}</ref> Other possible physiological effects include an enlarged [[clitoris]] and shallow [[vagina]].<ref>Richard D. McAnulty, M. Michele Burnette (2006) ''[http://books.google.com/books?id=KBi9aG0pQAkC&pg=PA165&lpg=PA165&dq=%22Digit+ratio%22+clitoris&source=bl&ots=SQjuEc1zm0&sig=VAVPJgd4f4wu6ToO9nPpGthPedY&hl=en&ei=hyZYTaLoKpKgtwfDxrnGDA&sa=X&oi=book_result&ct=result&resnum=1&ved=0CBMQ6AEwAA#v=onepage&q=%22Digit%20ratio%22%20clitoris&f=false Sex and sexuality, Volume 1]'', [[Greenwood Publishing Group]], p.165</ref>
* Males with CAH have more masculine (smaller) digit ratios than control males,<ref name=Brown2002/><ref name=Okten2002/> which also suggests that prenatal androgens affect digit ratios, since amniocentesis samples show that prenatal levels of testosterone are in the high normal range in males with CAH, while levels of the weaker androgen [[androstenedione]] are several fold higher than in control males.<ref>{{Cite journal|author=Pang S, Levine LS, Cederqvist LL, Fuentes M, Riccardi VM,Holcombe JH, Nitowsky HM, Sachs G, Anderson CE, Duchon MA,Owens R, Merkatz I, New MI|title=Amniotic fluid concentrations of delta5 and delta4 steroids in fetuses with congenital adrenal hyperplasia |journal=J Clin Endocrinol Metab |volume=51 |pages=223–229 |year=1980 |pmid=6447160 |doi=10.1210/jcem-51-2-223 |issue=2}}</ref><ref>{{Cite journal|author=Dorr, H. G., and Sippell, W. G. |title=Prenatal dexamethasone treatment in pregnancies at risk for congenital adrenal hyperplasia due to 21-hydroxylase deficiency: Effect on midgestational amniotic fluid steroid levels |journal=J. Clin. Endocrinol. Metab. |volume=76 |pages=117–120 |year=1993 |pmid=8421074 |doi=10.1210/jc.76.1.117 |issue=1}}</ref><ref>{{Cite journal|author=LWudy, S. A., Dorr, H. G., Solleder, C., Djalali, M., and Homoki, J. |title=Profiling steroid hormones in amniotic fluid of midpregnancy by routine stable isotope dilution/gas chromatography­ mass spectrometry: Reference values and concentrations in fetuses at risk for 21-hydroxylase deficiency |journal=J. Clin. Endocrinol. Metab. |volume=84 |pages=2724–2728 |year=1999 |pmid=10443667 |doi=10.1210/jc.84.8.2724 |issue=8 }}</ref> These measures indicate that males with CAH are exposed to greater prenatal concentrations of total androgens than are control males.
* Digit ratio in men correlates with genetic variation in the [[androgen receptor]] gene.<ref name="ReferenceA">{{Cite journal|doi=10.1016/S1090-5138(03)00052-7 |title=The second to fourth digit ratio and variation in the androgen receptor gene |year=2003 |first1=John T. |last1=Manning |first2=Peter E. |last2=Bundred |first3=Darren J. |last3=Newton |first4=Brian F. |last4=Flanagan |journal=Evolution and Human Behavior |volume=24 |pages=399–405|issue=6}}</ref> Men with genes that produce androgen receptors that are more sensitive to testosterone have lower, more masculine, digit ratios.
* XY individuals with [[androgen insensitivity syndrome]] (AIS) due to a dysfunctional gene for the androgen receptor present as women and have feminine digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding also demonstrates that the sex difference in digit ratios is unrelated to the Y chromosome per se.<ref>{{Cite journal|author=Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S |title=Fingers as a Marker of Prenatal Androgen Exposure |journal=Endocrinology |volume=150 |issue=11 |pages=5119–24 |year=2009 |month=November |pmid=19819951 |doi=10.1210/en.2009-0774 |pmc=2775980}}</ref>
* The sex difference in 2D:4D is present before birth in humans,<ref>{{Cite journal|author=Malas MA, Dogan S, Evcil EH, Desdicioglu K.|title=Fetal development of the hand, digits and digit ratio (2D:4D) |journal=Early Hum Dev |volume=82 |pages=469–475 |year=2006 |pmid=16473482 |doi=10.1016/j.earlhumdev.2005.12.002 |issue=7}}</ref><ref>{{Cite journal|author=Galis F, Ten Broek CM, Van Dongen S, Wijnaendts LC |title=Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D) |journal=Arch Sex Behav |volume=38 |pages=57–62 |year=2009 |pmid=19301112 |issue=1 |doi=10.1007/s10508-009-9485-7 |pmc=2811245}}</ref> which rules out any social influences that might affect digit growth differentially in the two sexes. Because all somatic sex differences in mammals to date have been found to be due to either androgenic masculinization or effects of the sex chromosomes, and as the AIS finding rules out a role for sex chromosomes in the sex difference in digit ratios, the prenatal sexual dimorphism also indicates that androgens act before birth to affect digit ratios.
* The ratio of testosterone to [[estradiol]] measured in 33 [[amniocentesis]] samples correlates with the child's subsequent 2D:4D ratio.<ref>{{Cite journal|author=Lutchmaya S, Baron-Cohen S, Raggatt P, Knickmeyer R, Manning JT |title=2nd to 4th digit ratios, fetal testosterone and estradiol |journal=Early Human Development |volume=77 |issue=1–2 |pages=23–8 |year=2004 |month=April |pmid=15113628 |doi=10.1016/j.earlhumdev.2003.12.002}}</ref>
* In [[pheasant]]s, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the [[Egg (biology)|egg]].<ref name=Romano2006>{{Cite journal|author=Romano M, Leoni B, Saino N |title=Examination marks of male university students positively correlate with finger length ratios (2D:4D) |journal=Biological Psychology |volume=71 |issue=2 |pages=175–82 |year=2006 |month=February |pmid=15978716 |doi=10.1016/j.biopsycho.2005.03.006}}</ref>


2D:4D digit ratios have been used alongside other methods in an attempt to understand Palaeolithic [[hand stencil]]s found in prehistoric European and Indonesian [[cave painting]].<ref>{{cite journal |doi=10.1017/S0003598X00093704 |title=Sexual dimorphism in Upper Palaeolithic hand stencils |journal=Antiquity |volume=80 |issue=308 |pages=390–404 |year=2006 |vauthors = Snow DR |s2cid=16301395 }}</ref><ref>{{cite journal |vauthors=Chazine JM, Noury A |title=Sexual Determination of Hand Stencils on the Main Panel of the Gua Masri II Cave (East-Kalimantan/Borneo – Indonesia) |journal=International Newsletter on Rock Art |volume=28 |pages=16–22 |url=https://hal.archives-ouvertes.fr/hal-00342551 |year=2008 |access-date=30 January 2019 |archive-date=31 January 2019 |archive-url=https://web.archive.org/web/20190131093148/https://hal.archives-ouvertes.fr/hal-00342551 |url-status=live }}</ref><ref name="Nelson_2006">{{cite journal | vauthors = Nelson E, Manning J, Sinclair A |doi=10.3828/bfarm.2006.1.6 |title=News Using the length of the 2nd to 4th digit ratio (2D:4D) to sex cave art hand stencils: Factors to consider |journal=Before Farming |volume=2006 |pages=1–7 |year=2006 }}</ref>
There is evidence that this reflects [[fetus|fetal]] exposure to the [[hormone]]s [[testosterone]]<ref>{{Cite journal|author=McIntyre MH |title=The use of digit ratios as markers for perinatal androgen action |journal=Reproductive Biology and Endocrinology |volume=4 |issue= |page=10 |year=2006 |pmid=16504142 |pmc=1409789 |doi=10.1186/1477-7827-4-10}}</ref> and [[estrogen]].{{Citation needed|date=October 2009}}


==Sexual dimorphism and age==
Several studies present evidence that digit ratios are heritable.<ref name=Paul2006>{{Cite journal|author=Paul SN, Kato BS, Hunkin JL, Vivekanandan S, Spector TD |title=The Big Finger: the second to fourth digit ratio is a predictor of sporting ability in women |journal=British Journal of Sports Medicine |volume=40 |issue=12 |pages=981–3 |year=2006 |month=December |pmid=17008344 |doi=10.1136/bjsm.2006.027193 |pmc=2577466}}</ref><ref>{{Cite journal|author=Gobrogge, K.L., S.M.Breedlove & K.L.Klump |title=Genetic and environmental influences on 2d:4d finger length ratios: a study of monozygotic and dizygotic male and female twins |journal=Archives Sexual Behavior |volume=37 |pages=112–118 |year=2008 |pmid=18074216 |doi=10.1007/s10508-007-9272-2 |issue=1}}</ref>
Studies measuring the 2D:4D of fetuses have found the ratio to be [[sexually dimorphic]], being lower in males than females.<ref name="Malas_2006" /><ref name="Galis_2010">{{cite journal | vauthors = Galis F, Ten Broek CM, Van Dongen S, Wijnaendts LC | title = Sexual dimorphism in the prenatal digit ratio (2D:4D) | journal = Archives of Sexual Behavior | volume = 39 | issue = 1 | pages = 57–62 | date = February 2010 | pmid = 19301112 | pmc = 2811245 | doi = 10.1007/s10508-009-9485-7 }}</ref>


However, when examining the extent of sexual dimorphism evident after birth, adults were found to demonstrate more dimorphism than children, who show the same extent of dimorphism as fetuses.<ref>{{Cite journal |last1=Galis |first1=Frietson |last2=Ten Broek |first2=Clara M. A. |last3=Van Dongen |first3=Stefan |last4=Wijnaendts |first4=Liliane C. D. |date=February 2010 |title=Sexual Dimorphism in the Prenatal Digit Ratio (2D:4D) |journal=Archives of Sexual Behavior |language=en |volume=39 |issue=1 |pages=57–62 |doi=10.1007/s10508-009-9485-7 |issn=0004-0002 |pmc=2811245 |pmid=19301112}}</ref> This has led the authors to suggest that digit ratio is also affected by postnatal development. Similarly, a study of people from Poland found statistically significant variation in 2D:4D between age groups (children, young adults and adults) for both males and females in the left hand, but only for women with the right hand.<ref>{{Cite journal |last1=Pruszkowska-Przybylska |first1=Paulina |last2=Kobus |first2=Magdalena |last3=Żądzińska |first3=Elżbieta |last4=Rosset |first4=Iwona |last5=Pruszkowska |first5=Milena |last6=Kuczyński |first6=Wojciech |last7=Sitek |first7=Aneta |date=2023-12-21 |title=The age difference in 2D:4D among the Polish population: An exploratory study |url=https://czasopisma.uni.lodz.pl/ar/article/view/18751 |journal=Anthropological Review |language=en |volume=86 |issue=4 |pages=99–110 |doi=10.18778/1898-6773.86.4.08 |issn=2083-4594 |doi-access=free |access-date=25 December 2023 |archive-date=3 January 2024 |archive-url=https://web.archive.org/web/20240103094435/https://czasopisma.uni.lodz.pl/ar/article/view/18751 |url-status=live |hdl=11089/49401 |hdl-access=free }}</ref>
The level of estrogen in the amniotic fluid is not correlated with higher 2D:4D, and when examined researchers found no difference in estrogen levels between males and females.<ref>{{cite journal | author = Lutchmaya S., Baron-Cohen S., Raggatt P., Knickmeyer R., Manning J. T. | year = 2004 | title = 2nd To 4th Digit Ratios, Fetal Testosterone and Estradiol | url = | journal = Early human development | volume = 77 | issue = 1-2| pages = 23–8 | doi = 10.1016/j.earlhumdev.2003.12.002 | pmid=15113628}}</ref>


Sexual dimorphism is strongest in digit ratios involving digit 2 with palmar measurements.<ref>{{cite journal | vauthors = Loehlin JC, Medland SE, Martin NG | title = Relative finger lengths, sex differences, and psychological traits | journal = Archives of Sexual Behavior | volume = 38 | issue = 2 | pages = 298–305 | date = April 2009 | pmid = 18340520 | doi = 10.1007/s10508-007-9303-z | s2cid = 12340685 }}</ref><ref name = "McFadden_2002">{{cite journal | vauthors = McFadden D, Shubel E | title = Relative lengths of fingers and toes in human males and females | journal = Hormones and Behavior | volume = 42 | issue = 4 | pages = 492–500 | date = December 2002 | pmid = 12488115 | doi = 10.1006/hbeh.2002.1833 | s2cid = 1360679 }}</ref><ref>{{cite journal | vauthors = Voracek M | title = Comparative study of digit ratios (2D:4D and other) and novel measures of relative finger length: testing magnitude and consistency of sex differences across samples | journal = Perceptual and Motor Skills | volume = 108 | issue = 1 | pages = 83–93 | date = February 2009 | pmid = 19425449 | doi = 10.2466/pms.108.1.83-93 | s2cid = 10076143 }}</ref> However, when measured dorsally, digit ratios involving digit 5 showed strongest dimorphism, with women having shorter fifth digits on average.<ref name="Kumar_2017"/> Overall, dorsal digit ratios demonstrate stronger sexual dimorphism than palmar digit ratios,<ref name="Kumar_2017"/> including the palmar 2D:4D ratio that has been the basis of most study.<ref>{{cite journal | vauthors = Hönekopp J, Watson S | title = Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand | journal = American Journal of Human Biology | volume = 22 | issue = 5 | pages = 619–630 | date = September 2010 | pmid = 20737609 | doi = 10.1002/ajhb.21054 | s2cid = 29503255 }}</ref> Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio.<ref name="Auger_2011"/> Thus, while most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.
==Explanation of the digit ratio effect==
It is not clear why digit ratio ought to be influenced by prenatal hormones. There is evidence of other similar traits, e.g. [[otoacoustic emissions]] and arm-to-trunk length ratio, which show similar effects. [[Hox genes]] responsible for both digit and penis development<ref>{{Cite journal|author=Dickman S. |title=HOX gene links limb, genital defects
|journal=Science |volume=275 |issue=5306 |pages=1568–9 |year=1997 |month=Mar |pmid= 9072822 |doi=10.1126/science.275.5306.1568}}</ref> have been implicated in this [[pleiotropy]]. Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes. Likewise, it is unclear why digit ratio on the right hand should be more responsive than that on the left hand, as is indicated by the greater sex difference on the right than the left.<ref name=Honekopp2010>{{Cite journal|author=Honekopp J, Watson S |title=Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand |journal=American Journal of Human Biology |volume=online |issue=5 |pages=619–30 |year=2010 |pmid=20737609 |doi=10.1002/ajhb.21054}}</ref>


A 2009 study in ''[[Biology Letters]]'' argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common [[allometric]] line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women",<ref>{{cite journal | vauthors = Kratochvíl L, Flegr J | title = Differences in the 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line | journal = Biology Letters | volume = 5 | issue = 5 | pages = 643–646 | date = October 2009 | pmid = 19553247 | pmc = 2781964 | doi = 10.1098/rsbl.2009.0346 }}</ref> which the authors claim may be the basis for the sex differences in 2D:4D and/or any putative hormonal influence on the ratios.
==Geographic and ethnic variation in 2D:4D==
Manning and colleagues have shown that 2D:4D ratios vary greatly between different ethnic groups.<ref name=Manning2000>{{Cite journal|author=Manning JT |title=The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success. evidence for sexually antagonistic genes? |journal=Evolution and Human Behavior |volume=21 |issue=3 |pages=163–183 |year=2000 |month=May |pmid=10828555 |doi=10.1016/S1090-5138(00)00029-5|author-separator=,|author2=Barley L|author3=Walton J|display-authors=3|last4=Lewis-Jones|first4=D.I.|last5=Trivers|first5=R.L.|last6=Singh|first6=D.|last7=Thornhill|first7=R.|last8=Rohde|first8=P.|last9=Bereczkei|first9=T.}}</ref><ref>{{Cite journal|author=Manning JT, Stewart A, Bundred PE, Trivers RL |title=Sex and ethnic differences in 2nd to 4th digit ratio of children |journal=Early Human Development |volume=80 |issue=2 |pages=161–8 |year=2004 |month=November |pmid=15500996 |doi=10.1016/j.earlhumdev.2004.06.004}}</ref> This variation is far larger than the differences between sexes; in Manning's words, "There’s more difference between a Pole and a Finn than a man and a woman."<ref name="Terrance 455–456">{{Cite journal
| last = Terrance | first = J. | coauthors = Williams, Michelle E. Pepitone, Scott E. Christensen, Bradley M. Cooke, Andrew D. Huberman, Nicholas J. Breedlove, Tessa J. Breedlove, Cynthia L. Jordan and S. Marc Breedlove
| title = Finger-length ratios and sexual orientation
| journal = Nature | issue = 6777| pages = 455–456 | date = 30 March 2000
| doi=10.1038/35006555
| url = http://www.nature.com/nature/journal/v404/n6777/abs/404455a0.html
| pmid = 10761903
| volume = 404
}}, available on-line at {{cite web
| title = Finger-length ratios and sexual orientation
| url = http://www.unl.edu/rhames/courses/readings/homofinger/homo_finger.html
| publisher = University of Nebraska-Lincoln
}} (quoted from New Scientist)</ref>


There has been no reliable correlation reported between 2D:4D and sex hormone levels in adulthood.<ref name="Hönekopp_2007">{{cite journal | vauthors = Hönekopp J, Bartholdt L, Beier L, Liebert A | title = Second to fourth digit length ratio (2D:4D) and adult sex hormone levels: new data and a meta-analytic review | journal = Psychoneuroendocrinology | volume = 32 | issue = 4 | pages = 313–321 | date = May 2007 | pmid = 17400395 | doi = 10.1016/j.psyneuen.2007.01.007 | s2cid = 44238240 }}</ref><ref name="No relationship between the digit r"/> Marta Kowal has suggested that testosterone levels are correlated with other factors (such as smoking, body weight and diet) that would impact the relationship between 2D:4D and an adult's testosterone level.<ref name="No relationship between the digit r"/>
==Correlation between digit ratio and traits==


==Effect of prenatal hormones==
Some authors suggest that digit ratio correlates with health, behavior, and even [[human sexuality|sexuality]] in later life. Below is a non-exhaustive list of some traits that have been either demonstrated or suggested to correlate with high digit ratio .
It has been proposed that 2D:4D ratio is affected by [[fetus|fetal]] exposure to [[sex hormone]]s, in particular to [[testosterone]] and other [[androgen]]s. Lower 2D:4D is found to correlate with higher prenatal androgen exposure.<ref name="Zheng_2011">{{cite journal | vauthors = Zheng Z, Cohn MJ | title = Developmental basis of sexually dimorphic digit ratios | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 39 | pages = 16289–16294 | date = September 2011 | pmid = 21896736 | pmc = 3182741 | doi = 10.1073/pnas.1108312108 | doi-access = free }}</ref><ref name="Hönekopp_2007"/><ref name="Brown_2002" /><ref name="Okten_2002" /><ref>{{cite journal | vauthors = Rivas MP, Moreira LM, Santo LD, Marques AC, El-Hani CN, Toralles MB | title = New studies of second and fourth digit ratio as a morphogenetic trait in subjects with congenital adrenal hyperplasia | journal = American Journal of Human Biology | volume = 26 | issue = 4 | pages = 559–561 | year = 2014 | pmid = 24668932 | doi = 10.1002/ajhb.22545 | s2cid = 7711519 }}</ref><ref name="Manning_2013">{{cite journal | vauthors = Manning JT, Kilduff LP, Trivers R | title = Digit ratio (2D:4D) in Klinefelter's syndrome | journal = Andrology | volume = 1 | issue = 1 | pages = 94–99 | date = January 2013 | pmid = 23258636 | doi = 10.1111/j.2047-2927.2012.00013.x | s2cid = 4503281 }}</ref><ref>{{cite journal | vauthors = Ventura T, Gomes MC, Pita A, Neto MT, Taylor A | title = Digit ratio (2D:4D) in newborns: influences of prenatal testosterone and maternal environment | journal = Early Human Development | volume = 89 | issue = 2 | pages = 107–112 | date = February 2013 | pmid = 23017880 | doi = 10.1016/j.earlhumdev.2012.08.009 | hdl = 10400.17/772 | hdl-access = free }}</ref><ref>{{cite journal | vauthors = McIntyre MH | title = The use of digit ratios as markers for perinatal androgen action | journal = Reproductive Biology and Endocrinology | volume = 4 | pages = 10 | date = February 2006 | pmid = 16504142 | pmc = 1409789 | doi = 10.1186/1477-7827-4-10 | doi-access = free }}</ref> Therefore digit ratio could be considered a [[proxy variable]] (indirect measure) for prenatal androgen exposure. Various studies suggest that 2D:4D is also influenced by prenatal [[estrogen]] exposure, and that it correlates negatively, not with prenatal testosterone alone, but with the testosterone-to-estrogen ratio.<ref name="Lutchmaya_2004">{{cite journal | vauthors = Lutchmaya S, Baron-Cohen S, Raggatt P, Knickmeyer R, Manning JT | title = 2nd to 4th digit ratios, fetal testosterone and estradiol | journal = Early Human Development | volume = 77 | issue = 1–2 | pages = 23–28 | date = April 2004 | pmid = 15113628 | doi = 10.1016/j.earlhumdev.2003.12.002 }}</ref><ref name="Mayhew_2007" /><ref name="Malas_2006">{{cite journal | vauthors = Malas MA, Dogan S, Evcil EH, Desdicioglu K | title = Fetal development of the hand, digits and digit ratio (2D:4D) | journal = Early Human Development | volume = 82 | issue = 7 | pages = 469–475 | date = July 2006 | pmid = 16473482 | doi = 10.1016/j.earlhumdev.2005.12.002 }}</ref><ref>{{cite journal | vauthors = Dean A, Sharpe RM | title = Clinical review: Anogenital distance or digit length ratio as measures of fetal androgen exposure: relationship to male reproductive development and its disorders | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 98 | issue = 6 | pages = 2230–2238 | date = June 2013 | pmid = 23569219 | doi = 10.1210/jc.2012-4057 | doi-access = free }}</ref>


Prenatal hormone exposure can be measured via [[amniocentesis]] (usually performed between 14th and 20th weeks of pregnancy), maternal [[blood test|serum sampling]] and umbilical cord sampling at birth, the latter being a measure of exposure in late [[gestation]]. A 2024 meta-analysis has suggested that digit ratio may be related to amniotic fluid testosterone levels, but not umbilical cord levels, requiring further validation.<ref>{{Cite journal |last1=Sorokowski |first1=Piotr |last2=Kowal |first2=Marta |date=5 October 2023 |title=Relationship between the 2D : 4D and prenatal testosterone, adult level testosterone, and testosterone change: Meta-analysis of 54 studies |url=https://onlinelibrary.wiley.com/doi/10.1002/ajpa.24852 |journal=American Journal of Biological Anthropology |volume=183 |issue=1 |pages=20–38 |language=en |doi=10.1002/ajpa.24852 |pmid=37795916 |s2cid=263670473 |issn=2692-7691}}</ref>
===Physiology and disease===
* Lowered sperm counts<ref>{{Cite journal|author=Manning JT, Scutt D, Wilson J, Lewis-Jones DI |title=The ratio of 2nd to 4th digit length: a predictor of sperm numbers and concentrations of testosterone, luteinizing hormone and oestrogen |journal=Human Reproduction |volume=13 |issue=11 |pages=3000–4 |year=1998 |month=November |pmid=9853845 |doi=10.1093/humrep/13.11.3000}}</ref>
* Increase risk for [[heart disease]]<ref>{{Cite journal|author=Manning JT, Bundred PE |title=The ratio of second to fourth digit length and age at first myocardial infarction in men: a link with testosterone? |journal=British Journal of Cardiology |volume=8 |issue=12 |pages=720–3 |year=2001 |issn=0969-6113}}</ref>
* Increased risk of [[obesity]] and the [[metabolic syndrome]]<ref>{{Cite journal|author=Fink B, Manning JT, Neave N |title=The 2nd-4th digit ratio (2D:4D) and neck circumference: implications for risk factors in coronary heart disease |journal=International Journal of Obesity |volume=30 |issue=4 |pages=711–4 |year=2006 |month=April |pmid=16261185 |doi=10.1038/sj.ijo.0803154}}</ref>
* Reduced risk for [[prostate cancer]]<ref name="prostate">{{cite news|last=Walsh|first=Fergus|title=Index finger length prostate cancer clue|url=http://www.bbc.co.uk/news/health-11880415|work=BBC News|accessdate=1 December 2010|date=1 December 2010}}</ref>
* 2D:4D and birth size in males<ref> Ronalds G, Phillips DIW, Godfrey KM, Manning JT. The ratio of second to fourth digit lengths: a marker of impaired fetal growth? Early Hum. Dev. 2002;68:21–6.</ref>


A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development".<ref name="Zheng_2011" /> The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood.<ref>{{cite journal | vauthors = Garn SM, Burdi AR, Babler WJ, Stinson S | title = Early prenatal attainment of adult metacarpal-phalangeal rankings and proportions | journal = American Journal of Physical Anthropology | volume = 43 | issue = 3 | pages = 327–332 | date = November 1975 | pmid = 1211429 | doi = 10.1002/ajpa.1330430305 | hdl = 2027.42/37557 | hdl-access = free }}</ref> If, during this period, the fetus is exposed to androgens (levels of which are usually far higher in male than female fetuses) the growth rate of the 4th digit is increased. In a 2006 study, digit ratio analysis of opposite-sex [[dizygotic twins]] found that the females in these pairings were born with significantly lower 2D:4D ratio, postulated to occur from exposure to excess androgens from their brothers in utero (the hormone-transfer theory).<ref>{{cite journal | vauthors = van Anders SM, Vernon PA, Wilbur CJ | title = Finger-length ratios show evidence of prenatal hormone-transfer between opposite-sex twins | journal = Hormones and Behavior | volume = 49 | issue = 3 | pages = 315–319 | date = March 2006 | pmid = 16143332 | doi = 10.1016/j.yhbeh.2005.08.003 | s2cid = 17288420 | hdl = 2027.42/83921 | hdl-access = free }}</ref> However, an attempt to replicate these findings with a larger sample of dizygotic twins (867 individuals) found no differences in the [[variance]] or [[co-variance]] of same-sex and opposite-sex pairings to support the theory, though it did confirm female 2D:4D to be significantly higher than male as expected.<ref>{{Cite journal |vauthors=Medland SE, Loehlin JC, Martin NG |date=1 April 2008 |title=No effects of prenatal hormone transfer on digit ratio in a large sample of same- and opposite-sex dizygotic twins |url=https://www.sciencedirect.com/science/article/pii/S0191886907004230 |journal=Personality and Individual Differences |language=en |volume=44 |issue=5 |pages=1225–1234 |doi=10.1016/j.paid.2007.11.017 |issn=0191-8869 |access-date=10 June 2023 |archive-date=28 November 2012 |archive-url=https://web.archive.org/web/20121128133338/http://www.sciencedirect.com/science/article/pii/S0191886907004230 |url-status=live }}</ref>
===Psychological disorders===


Researchers have raised concerns that, although the general trend points towards a correlation between digit ratio and early androgen exposure, many results have not been statistically significant.<ref name="Richards_2017">{{cite journal | vauthors = Richards G | title = Digit ratio (2D:4D) and prenatal/perinatal sex hormones: A response to Manning and Fink (2017) | journal = Early Human Development | volume = 113 | pages = 75–76 | date = October 2017 | pmid = 28917583 | doi = 10.1016/j.earlhumdev.2017.09.004 | url = https://eprint.ncl.ac.uk/fulltext.aspx?url=249530/77FA418E-6576-4475-A6B5-EEE33F7CAF94.pdf&pub_id=249530 }}</ref>
* Reduced rate of [[autism]] and [[Asperger syndrome]].<ref name="Manning">{{Cite journal|author=Manning JT, Baron-Cohen S, Wheelwright S, Sanders G |title=The 2nd to 4th digit ratio and autism |journal=Developmental Medicine and Child Neurology |volume=43 |issue=3 |pages=160–4 |year=2001 |month=March |pmid=11263685 |doi=10.1017/S0012162201000317}}</ref>
* Increased risk for [[Depression (mood)|depression]]<ref>{{cite journal|last=Bailey, A. & Hurd, P.|title=Depression in men is associated with more feminine finger length ratios|journal=Personality and Individual Differences|year=2005|volume=39|issue=4|pages=829–836|doi=10.1016/j.paid.2004.12.017|url=http://www.sciencedirect.com/science/article/pii/S0191886905001133}}</ref>
* Increase rate of [[schizophrenia]]<ref name="Arató M, Frecska E, Beck C, An M, Kiss H 2004 191–4">{{Cite journal|author=Arató M, Frecska E, Beck C, An M, Kiss H |title=Digit length pattern in schizophrenia suggests disturbed prenatal hemispheric lateralization |journal=Progress in Neuro-psychopharmacology & Biological Psychiatry |volume=28 |issue=1 |pages=191–4 |year=2004 |month=January |pmid=14687873 |doi=10.1016/j.pnpbp.2003.09.020}}</ref>
*Increased rate of [[ADHD]]<ref>{{Cite journal|author=McFadden, D., Westhafer, J.G., Pasanen, E.G., Carlson, C.L., and Tucker, D.M. |title=Physiological evidence of hypermasculinization in boys with the inattentive subtype of attention-deficit/hyperactivity disorder (ADHD) |journal=Clinical Neuroscience Research |volume=5 |pages=233–245 |year=2005 |doi=10.1016/j.cnr.2005.09.004|issue=5–6}}</ref><ref>{{Cite journal|author=Stevenson JC, Everson PM, Williams DC, Hipskind G, Grimes M, Mahoney ER. |title=Attention deficit/hyperactivity disorder (ADHD) symptoms and digit ratios in a college sample |journal=Am J Hum Biol |volume=19 |pages=41–50 |year=2007 |pmid=17160985 |doi=10.1002/ajhb.20571 |issue=1}}</ref><ref>{{Cite journal|author=Martel, M.M, K.L.Gobrogge, S.M.Breedlove & J.T.Nigg |title=Masculinized Finger-Length Ratios of Boys, but Not Girls, Are Associated With Attention-Deficit/Hyperactivity Disorder |journal=Behavioral Neuroscience |volume=122 |pages=273–281 |year=2008 |pmid=18410167 |pmc=2902868 |doi=10.1037/0735-7044.122.2.273 |issue=2}}</ref><ref>{{Cite journal|author=Martel, M.M. |title=Conscientiousness as a mediator of the association between masculinized finger-length ratios and attention-deficit/hyperactivity disorder (ADHD) |journal=J Child Psychol Psychiatry |volume=50 |pages=790–798. |year=2009 |pmid=19298468 |doi=10.1111/j.1469-7610.2009.02065.x |issue=7}}</ref>
*Reduced risk for [[eating disorders]] in females<ref name="Klump, K. L., Gobrogge, K. L., Perkins, P. S., Thorne, D., Sisk, C. L., Breedlove, S.M. 2006 539–546">{{Cite journal|author=Klump, K. L., Gobrogge, K. L., Perkins, P. S., Thorne, D., Sisk, C. L., Breedlove, S.M. |title=Preliminary evidence that gonadal hormones organize and activate disordered eating |journal=Psychol Med |volume=36 |pages=539–546 |year=2006 |pmid=16336745 |doi=10.1017/S0033291705006653 |issue=4}}</ref> and males<ref>{{Cite journal|author=Smith, A. R., Hawkeswood, S. E., Joiner, T. E. |title=The measure of a man: Associations between digit ratio and disordered eating in males |journal=Int J Eat Disord |volume=28 |issue=1 |pages=191–4 |year=2009 |pmid=19718667 |doi=10.1002/eat.20736}}</ref>
* Increased rate of [[psychopathy]] in females<ref>{{cite journal|last=Blanchard|first=A.|coauthors=Lyons, M.|title=An Investigation into the Relationship between Digit Length Ratio and Psychopathy|journal=British Journal of Forensic Practice|year=2010|month=May|volume=12|issue=2|url=http://lhu.academia.edu/AlysonBlanchard/Papers/305985/An_Investigation_Into_the_Relationship_Between_Digit_Length_Ratio_2D_4D_and_Psychopathy}}</ref>
* Reduced risk of [[alcohol dependency]]<ref>{{Cite journal|author=Kornhuber J, Erhard G, Lenz B, Kraus T, Sperling W, Bayerlein K, Biermann T, Stoessel C |title=Low Digit Ratio 2D∶4D in Alcohol Dependent Patients |journal=PLoS ONE |volume=6 |issue=4 |pages=e19332 |year=2011 |month=April |pmid=21547078 |doi=10.1371/journal.pone.0019332|editor1-last=Zhang|editor1-first=Xiang Yang|pmc=3081847}}</ref>
* Increased [[Anxiety]] in males<ref>{{cite journal|last=Evardone & Alexander|title=Anxiety, Sex-linked Behavior, and Digit Ratios|journal=Arch Sex Behav.|year=2009|volume=38|issue=3|pmc=2768336|pmid=17943431|doi=10.1007/s10508-007-9260-6|pages=442–55}}</ref>


Various findings have also challenged the general trend. One study of 66 children that attempted to replicate the findings of a frequently cited paper on the topic<ref name="Lutchmaya_2004" /> found no association between prenatal testosterone and estrogen levels and 2D:4D in childhood.<ref name="Richards_2021">{{cite journal | vauthors = Richards G, Browne WV, Constantinescu M | title = Digit ratio (2D:4D) and amniotic testosterone and estradiol: an attempted replication of Lutchmaya ''et al.'' (2004) | journal = Journal of Developmental Origins of Health and Disease | volume = 12 | issue = 6 | pages = 859–864 | date = December 2021 | pmid = 33472723 | doi = 10.1017/S2040174420001294 | s2cid = 231664924 | url = https://repository.uel.ac.uk/download/80146174549c10eed26a47263c60f382cfb6cac134adaa6fa3e297afd005044b/349843/Richards%2C%20Brown%20and%20Constantinescu.pdf | access-date = 23 March 2022 | archive-date = 2 April 2022 | archive-url = https://web.archive.org/web/20220402034729/https://repository.uel.ac.uk/download/80146174549c10eed26a47263c60f382cfb6cac134adaa6fa3e297afd005044b/349843/Richards%2C%20Brown%20and%20Constantinescu.pdf | url-status = live }}</ref> Another paper also found no relationship between 2D:4D ratios and umbilical cord androgen and estrogen levels.<ref name="research-repository.uwa.edu.au">{{cite journal | vauthors = Hollier LP, Keelan JA, Jamnadass ES, Maybery MT, Hickey M, Whitehouse AJ | title = Adult digit ratio (2D:4D) is not related to umbilical cord androgen or estrogen concentrations, their ratios or net bioactivity | journal = Early Human Development | volume = 91 | issue = 2 | pages = 111–117 | date = February 2015 | pmid = 25594498 | doi = 10.1016/j.earlhumdev.2014.12.011 | url = https://research-repository.uwa.edu.au/en/publications/b3d44281-0fcf-499c-9743-7daac128c486 }}</ref> A large meta-analysis studying genomic correlations was unable to find evidence for 2D:4D being a marker for prenatal androgen exposure, but did not exclude the possibility given constraints in genomic knowledge (with 3.8% of the variance in 2D:4D ratio accounted for genetically).<ref>{{cite journal | vauthors = Warrington NM, Shevroja E, Hemani G, Hysi PG, Jiang Y, Auton A, Boer CG, Mangino M, Wang CA, Kemp JP, McMahon G, Medina-Gomez C, Hickey M, Trajanoska K, Wolke D, Ikram MA, Montgomery GW, Felix JF, Wright MJ, Mackey DA, Jaddoe VW, Martin NG, Tung JY, Davey Smith G, Pennell CE, Spector TD, van Meurs J, Rivadeneira F, Medland SE, Evans DM | display-authors = 6 | title = Genome-wide association study identifies nine novel loci for 2D:4D finger ratio, a putative retrospective biomarker of testosterone exposure in utero | journal = Human Molecular Genetics | volume = 27 | issue = 11 | pages = 2025–2038 | date = June 2018 | pmid = 29659830 | pmc = 5961159 | doi = 10.1093/hmg/ddy121 }}</ref>
===Physical and competitive behavior===


===Developmental disorders===
* Reduced performance in sports <ref>{{Cite journal|author=Manning JT, Taylor RP |title=Second to fourth digit ratio and male ability in sport: implications for sexual selection in humans |journal=Evolution and Human Behavior |volume=22 |issue=1 |pages=61–69 |year=2001 |month=January |pmid=11182575 |doi=10.1016/S1090-5138(00)00063-5}}</ref>
Women with [[congenital adrenal hyperplasia]] (CAH), who have elevated androgen levels before birth, have lower (more masculine) 2D:4D on average<ref name="Brown_2002">{{cite journal | vauthors = Brown WM, Hines M, Fane BA, Breedlove SM | title = Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia | journal = Hormones and Behavior | volume = 42 | issue = 4 | pages = 380–386 | date = December 2002 | pmid = 12488105 | doi = 10.1006/hbeh.2002.1830 | s2cid = 8886238 }}</ref><ref name="Okten_2002">{{cite journal | vauthors = Okten A, Kalyoncu M, Yariş N | title = The ratio of second- and fourth-digit lengths and congenital adrenal hyperplasia due to 21-hydroxylase deficiency | journal = Early Human Development | volume = 70 | issue = 1–2 | pages = 47–54 | date = December 2002 | pmid = 12441204 | doi = 10.1016/s0378-3782(02)00073-7 }}</ref><ref name="Ciumas_2009">{{cite journal | vauthors = Ciumas C, Lindén Hirschberg A, Savic I | title = High fetal testosterone and sexually dimorphic cerebral networks in females | journal = Cerebral Cortex | volume = 19 | issue = 5 | pages = 1167–1174 | date = May 2009 | pmid = 18854582 | doi = 10.1093/cercor/bhn160 | doi-access = free }}</ref> along with other possible physiological effects such as an enlarged [[clitoris]] and shallow [[vagina]].<ref>{{cite book| vauthors = McAnulty RD, Burnette MM |year= 2006| url= https://books.google.com/books?id=KBi9aG0pQAkC&pg=PA165 |title= Sex and sexuality | volume = 1 | publisher= [[Greenwood Publishing Group]]| page= 165| isbn= 9780313049194}}</ref> Males with CAH also express lower digit ratios than [[case–control study|controls]].<ref name="Brown_2002" /><ref name="Okten_2002" /> Amniocentesis samples in males with CAH show that prenatal levels of testosterone are in the high-normal range and levels of the weaker androgen [[androstenedione]] are several fold higher than in controls,<ref name="Pang_1980">{{cite journal | vauthors = Pang S, Levine LS, Cederqvist LL, Fuentes M, Riccardi VM, Holcombe JH, Nitowsky HM, Sachs G, Anderson CE, Duchon MA, Owens R, Merkatz I, New MI | display-authors = 6 | title = Amniotic fluid concentrations of delta 5 and delta 4 steroids in fetuses with congenital adrenal hyperplasia due to 21 hydroxylase deficiency and in anencephalic fetuses | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 51 | issue = 2 | pages = 223–229 | date = August 1980 | pmid = 6447160 | doi = 10.1210/jcem-51-2-223 }}</ref><ref name="Dörr_1993">{{cite journal | vauthors = Dörr HG, Sippell WG | title = Prenatal dexamethasone treatment in pregnancies at risk for congenital adrenal hyperplasia due to 21-hydroxylase deficiency: effect on midgestational amniotic fluid steroid levels | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 76 | issue = 1 | pages = 117–120 | date = January 1993 | doi = 10.1210/jcem.76.1.8421074 | pmid = 8421074 }}</ref><ref name="Wudy_1999">{{cite journal | vauthors = Wudy SA, Dörr HG, Solleder C, Djalali M, Homoki J | title = Profiling steroid hormones in amniotic fluid of midpregnancy by routine stable isotope dilution/gas chromatography-mass spectrometry: reference values and concentrations in fetuses at risk for 21-hydroxylase deficiency | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 84 | issue = 8 | pages = 2724–2728 | date = August 1999 | doi = 10.1210/jcem.84.8.5870 | pmid = 10443667 | doi-access = free }}</ref> indicating that males with CAH are exposed to greater prenatal concentrations of total androgens.
* Reduced financial trading ability<ref>{{Cite journal|author=Coates JM, Gurnell M, Rustichini A |title=Second-to-fourth digit ratio predicts success among high-frequency financial traders |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=106 |issue=2 |pages=623–8 |year=2009 |month=January |pmid=19139402 |pmc=2626753 |doi=10.1073/pnas.0810907106 |laysummary=http://www.time.com/time/business/article/0,8599,1871066,00.html |laysource=[[Time (magazine)|Time]] |laydate=12 January 2009}}</ref>
* Right handedness Skills<ref>{{Cite journal|author=Fink B, Manning JT, Neave N, Tan U |title=Second to fourth digit ratio and hand skill in Austrian children |journal=Biological Psychology |volume=67 |issue=3 |pages=375–84 |year=2004 |month=November |pmid=15294393 |doi=10.1016/j.biopsycho.2004.03.012}}</ref>


A greater (more feminine) digit ratio occurs for men with [[Klinefelter's syndrome]], who have reduced testosterone secretion throughout life compared to their fathers or to controls.<ref name=Manning_2013/>
===Cognition and personality===


Digit ratio in men may correlate with genetic variation in the [[androgen receptor]] gene.<ref name="Manning_2003">{{Cite journal |doi=10.1016/S1090-5138(03)00052-7 |title=The second to fourth digit ratio and variation in the androgen receptor gene |year=2003 |vauthors = Manning JT, Bundred PE, Newton DJ, Flanagan BF |journal=Evolution and Human Behavior |volume=24 |pages=399–405 |issue=6 |bibcode=2003EHumB..24..399M }}</ref> Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more [[CAG repeats]]) have greater digit ratios, though there have also been reports of failure in replicating this finding.<ref>{{Cite journal| vauthors = Hampson E, Sankar JS |title=Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D:4D ratio |journal=Evol Hum Behav |volume=33 | issue= 5|pages=557–561 |year=2012 |doi=10.1016/j.evolhumbehav.2012.02.003|bibcode=2012EHumB..33..557H }}</ref> Men with less sensitive androgen receptors may compensate for this by secreting more testosterone via reduced inhibitory feedback on [[gonadotropins]].<ref>{{cite journal | vauthors = Crabbe P, Bogaert V, De Bacquer D, Goemaere S, Zmierczak H, Kaufman JM | title = Part of the interindividual variation in serum testosterone levels in healthy men reflects differences in androgen sensitivity and feedback set point: contribution of the androgen receptor polyglutamine tract polymorphism | journal = The Journal of Clinical Endocrinology and Metabolism | volume = 92 | issue = 9 | pages = 3604–3610 | date = September 2007 | pmid = 17579205 | doi = 10.1210/jc.2007-0117 | s2cid = 33761349 | doi-access = free }}</ref> Thus, it is not clear that 2D:4D would be expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen.
* [[Assertiveness]] in women<ref name=Wilson1983/>
* [[Aggression]]<ref name="Bailey & Hurd 2005"/><ref>{{Cite journal|author=Benderlioglu Z, Nelson RJ |title=Digit length ratios predict reactive aggression in women, but not in men |journal=Hormones and Behavior |volume=46 |issue=5 |pages=558–64 |year=2004 |month=December |pmid=15555497 |doi=10.1016/j.yhbeh.2004.06.004}}</ref>
* Masculinity of Handwriting<ref>{{Cite journal|doi=10.1016/j.paid.2005.01.024 |title=Do differences in sex hormones affect handwriting style? Evidence from digit ratio and sex role identity as determinants of the sex of handwriting |month=July |year=2005 |first1=John R. |last1=Beech |first2=Isla C. |last2=Mackintosh |journal=Personality and Individual Differences |volume=39 |issue=2 |pages=459–68}}</ref>
* Perceived 'dominance' and masculinity of man's face<ref>{{Cite journal|author=Neave N, Laing S, Fink B, Manning JT |title=Second to fourth digit ratio, testosterone and perceived male dominance |journal=Proceedings. Biological Sciences |volume=270 |issue=1529 |pages=2167–72 |year=2003 |month=October |pmid=14561281 |pmc=1691489 |doi=10.1098/rspb.2003.2502}}</ref><ref>{{Cite journal|author=Burriss RP, Little AC, Nelson EC |title=2D:4D and sexually dimorphic facial characteristics |journal=Archives of Sexual Behavior |volume=36 |issue=3 |pages=377–84 |year=2007 |month=June |pmid=17203400 |doi=10.1007/s10508-006-9136-1}}</ref>
* [[Personality]]<ref>{{Cite journal|doi=10.1016/S0191-8869(02)00002-8 |title=A preliminary investigation of the associations between personality, cognitive ability and digit ratio |month=November |year=2002 |first1=Elizabeth J. |last1=Austin |first2=John T. |last2=Manning |first3=Katherine |last3=McInroy |first4=Elizabeth |last4=Mathews |journal=Personality and Individual Differences |volume=33 |issue=7 |pages=1115–24}}</ref><ref>Fink et al. 2004{{Verify source|which one?? there are two in the reference section!|date=October 2009}}</ref><ref>{{Cite journal|doi=10.1016/j.paid.2005.03.016 |title=Second-to-fourth digit ratio related to Verbal and Numerical Intelligence and the Big Five |month=October |year=2005 |first1=Marc F. |last1=Luxen |first2=Bram P. |last2=Buunk |journal=Personality and Individual Differences |volume=39 |issue=5 |pages=959–66}}</ref>
* Exam scores: a higher ratio is correlated with higher exam scores among male students<ref name=Romano2006/><ref>{{Cite journal|author=Brosnan MJ |title=Digit ratio as an indicator of numeracy relative to literacy in 7-year-old British schoolchildren |journal=British Journal of Psychology |volume=99 |issue=Pt 1 |pages=75–85 |year=2008 |month=February |pmid=17535470 |doi=10.1348/000712607X197406 |laysummary=http://www.livescience.com/7290-finger-length-predicts-sat-performance.html |laysource=[[LiveScience]] |laydate=22 May 2007}}</ref>
* Musical ability<ref>{{Cite journal|doi=10.1016/S1090-5138(99)00026-4 |title=Second to fourth digit ratio in elite musicians Evidence for musical ability as an honest signal of male fitness |month=January |year=2000 |last1=Sluming |first1=Vanessa A. |last2=Manning |first2=John T. |journal=Evolution and Human Behavior |volume=21 |issue=1 |pages=1–9}}</ref>


XY individuals with [[androgen insensitivity syndrome]] due to a dysfunctional gene for the androgen receptor present as women and have greater digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding suggests that the sex difference in digit ratios may be unrelated to the [[Y chromosome]] per se.<ref>{{cite journal | vauthors = Berenbaum SA, Bryk KK, Nowak N, Quigley CA, Moffat S | title = Fingers as a marker of prenatal androgen exposure | journal = Endocrinology | volume = 150 | issue = 11 | pages = 5119–5124 | date = November 2009 | pmid = 19819951 | pmc = 2775980 | doi = 10.1210/en.2009-0774 }}</ref>
===Management===


==Geographic and ethnic variation==
* Leadership<ref>Derval, Diana. ''The Right Sensory Mix: Targeting Consumer Product Development Scientifically''. Springer, 2010, p. 129-130.</ref>
Several studies have presented evidence that digit ratios are heritable.<ref name="Paul_2006">{{cite journal | vauthors = Paul SN, Kato BS, Hunkin JL, Vivekanandan S, Spector TD | title = The big finger: the second to fourth digit ratio is a predictor of sporting ability in women | journal = British Journal of Sports Medicine | volume = 40 | issue = 12 | pages = 981–983 | date = December 2006 | pmid = 17008344 | pmc = 2577466 | doi = 10.1136/bjsm.2006.027193 }}</ref><ref>{{cite journal | vauthors = Gobrogge KL, Breedlove SM, Klump KL | title = Genetic and environmental influences on 2D:4D finger length ratios: a study of monozygotic and dizygotic male and female twins | journal = Archives of Sexual Behavior | volume = 37 | issue = 1 | pages = 112–118 | date = February 2008 | pmid = 18074216 | doi = 10.1007/s10508-007-9272-2 | author2-link = Marc Breedlove | s2cid = 41921265 }}</ref>
* Innovation<ref>Derval, Diana. ''The Right Sensory Mix: Targeting Consumer Product Development Scientifically''. Springer, 2010, p. 129-135.</ref>


Manning and colleagues have reported that 2D:4D ratios vary greatly between different ethnic groups. In a study of [[Han Chinese|Han]], [[Berbers|Berber]], [[Uyghurs|Uygur]] and [[Jamaicans|Jamaican]] children, Manning ''et al.'' found that Han children had the highest mean values of 2D:4D (0.954±0.032). They were followed by the Berbers (0.950±0.033), then the Uygurs (0.946±0.037) and the Jamaican children had the lowest mean 2D:4D (0.935±0.035).<ref name="Manning_2000">{{cite journal | vauthors = Manning JT, Barley L, Walton J, Lewis-Jones DI, Trivers RL, Singh D, Thornhill R, Rohde P, Bereczkei T, Henzi P, Soler M, Szwed A | display-authors = 6 | title = The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success. evidence for sexually antagonistic genes? | journal = Evolution and Human Behavior | volume = 21 | issue = 3 | pages = 163–183 | date = May 2000 | pmid = 10828555 | doi = 10.1016/S1090-5138(00)00029-5 | bibcode = 2000EHumB..21..163M }}</ref><ref name="Manning_2004">{{cite journal | vauthors = Manning JT, Stewart A, Bundred PE, Trivers RL | title = Sex and ethnic differences in 2nd to 4th digit ratio of children | journal = Early Human Development | volume = 80 | issue = 2 | pages = 161–168 | date = November 2004 | pmid = 15500996 | doi = 10.1016/j.earlhumdev.2004.06.004 }}</ref> This variation is far larger than the differences between sexes; in Manning's words, "There's more difference between a [[Polish people|Pole]] and a [[Finnish people|Finn]], than a man and a woman."<ref name=pmid10761903>{{cite journal | vauthors = Williams TJ, Pepitone ME, Christensen SE, Cooke BM, Huberman AD, Breedlove NJ, Breedlove TJ, Jordan CL, Breedlove SM | display-authors = 6 | title = Finger-length ratios and sexual orientation | journal = Nature | volume = 404 | issue = 6777 | pages = 455–456 | date = March 2000 | pmid = 10761903 | doi = 10.1038/35006555 | s2cid = 205005405 | bibcode = 2000Natur.404..455W }}</ref>
===Sensory Perception===


In 2007, Manning ''et al.'' reported further evidence of ethnic variation in mean 2D:4D, with higher ratios for Whites, non-Chinese Asians and Middle-Easterners, and lower ratios in Chinese and Black samples.<ref>{{cite journal | vauthors = Manning JT, Churchill AJ, Peters M | title = The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D) | journal = Archives of Sexual Behavior | volume = 36 | issue = 2 | pages = 223–233 | date = April 2007 | pmid = 17373585 | doi = 10.1007/s10508-007-9171-6 | s2cid = 2042042 }}</ref> A 2008 study by Lu ''et al.'' found that the mean values of 2D:4D of the [[Hui people|Hui]] and the Han in [[Ningxia]] were lower than those in European countries like Britain.<ref>{{Cite journal | vauthors = Lu H, Huo ZH, Shi ZY, Peng L, Dang J, Jiao HY, Zhu YS, Zhong HJ, Chen YT | display-authors = 6 | title = Study on the digit ratio of Hui and Han ethnic groups in Ningxia. | journal = Acta Anatomica Sinica | date = 2008 | volume = 39 | issue = 2 | pages = 267 | url = http://en.cnki.com.cn/Article_en/CJFDTOTAL-JPXB200802033.htm | access-date = 7 November 2016 | archive-date = 7 November 2016 | archive-url = https://web.archive.org/web/20161107221950/http://en.cnki.com.cn/Article_en/CJFDTOTAL-JPXB200802033.htm | url-status = dead }}</ref>
* Smell perception<ref>Derval, Diana. ''The Right Sensory Mix: Targeting Consumer Product Development Scientifically''. Springer, 2010, p. 62-67.</ref>
* Color perception<ref>Derval, Diana. ''The Right Sensory Mix: Targeting Consumer Product Development Scientifically''. Springer, 2010, p. 112-122.</ref>
* Tactile perception<ref>Derval, Diana (2011). "Hormonal Quotient and tactile sensitivity: a segmentation model to understand and predict individuals' texture preferences based on prenatal exposure to hormones". Proceedings of Society for Behavioral Neuroendocrinology 15th Annual Meeting, Queretaro, Mexico, p.125.</ref>


Two studies explored the question of whether geographical differences in 2D:4D ratios were caused by gene pool differences or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). The conclusions were that geographical differences in 2D:4D ratio were caused by genetic pool differences, not by geographical latitude.<ref>{{cite journal | vauthors = Loehlin JC, McFadden D, Medland SE, Martin NG | title = Population differences in finger-length ratios: ethnicity or latitude? | journal = Archives of Sexual Behavior | volume = 35 | issue = 6 | pages = 739–742 | date = December 2006 | pmid = 16688484 | doi = 10.1007/s10508-006-9039-1 | s2cid = 4763807 | citeseerx = 10.1.1.579.6269 }}</ref><ref>{{cite journal | vauthors = Xu Y, Zheng Y | title = The digit ratio (2D:4D) in China: A meta-analysis | journal = American Journal of Human Biology | volume = 27 | issue = 3 | pages = 304–309 | date = 6 May 2015 | pmid = 25284473 | doi = 10.1002/ajhb.22639 | s2cid = 40725875 }}</ref>
===Sexual orientation===


[[Consanguinous|Consanguineous]] parentage ([[inbreeding]]) has been found to lower the 2D:4D ratio in offspring,<ref>{{cite journal | vauthors = Ozener B, Hurd PL, Duyar I | title = Inbreeding is associated with lower 2D: 4D digit ratio | journal = American Journal of Human Biology | volume = 26 | issue = 2 | pages = 183–188 | year = 2014 | pmid = 24288299 | doi = 10.1002/ajhb.22491 | s2cid = 23618195 }}</ref> which may account for some of the geographical and ethnic variation in 2D:4D ratios, as consanguinity rates depend on, among others, religion, culture, and geography.<ref>{{cite journal | vauthors = Tadmouri GO, Nair P, Obeid T, Al Ali MT, Al Khaja N, Hamamy HA | title = Consanguinity and reproductive health among Arabs | journal = Reproductive Health | volume = 6 | pages = 17 | date = October 2009 | pmid = 19811666 | pmc = 2765422 | doi = 10.1186/1742-4755-6-17 | doi-access = free }}</ref>
* Bem sex role score in women;<ref name=Csatho2003>{{Cite journal|author=Csathó A, Osváth A, Bicsák E, Karádi K, Manning J, Kállai J |title=Sex role identity related to the ratio of second to fourth digit length in women |journal=Biological Psychology |volume=62 |issue=2 |pages=147–56 |year=2003 |month=February |pmid=12581689 |doi=10.1016/S0301-0511(02)00127-8}}</ref> erotic role preferences in gay men.<ref>{{Cite journal|author=McIntyre MH |title=Digit ratios, childhood gender role behavior, and erotic role preferences of gay men |journal=Archives of Sexual Behavior |volume=32 |issue=6 |pages=495–6 |year=2003 |month=December |pmid=14627046 |doi=10.1023/A:1026054625638}}</ref>
* Lesbians vs. straight women;<ref name=Csatho2003/><ref name = "Williams, T.J. et al. (2000)">{{Cite journal|author=Williams TJ |title=Finger-length ratios and sexual orientation |journal=Nature |volume=404 |issue=6777 |pages=455–6 |year=2000 |month=March |pmid=10761903 |doi=10.1038/35006555 |url=http://msu.edu/~breedsm/pdf/breedlove2000.pdf|author-separator=,|author2=Pepitone ME|author3=Christensen SE|display-8authors=3|last4=Cooke|first4=Bradley M.|last5=Huberman|first5=Andrew D.|last6=Breedlove|first6=Nicholas J.|last7=Breedlove|first7=Tessa J.|last8=Jordan|first8=Cynthia L.|last9=Breedlove|first9=S. Marc}}</ref><ref name = "Tortorice, J.L. (2002)">{{Cite journal|author=Tortorice JL |year=2002 |publisher=Rutgers University |title=Written on the body: butch/femme lesbian gender identity and biological correlates |oclc=80234273}}</ref><ref name = "McFadden and Shubel (2002)">{{Cite journal|author=McFadden D, Shubel E |title=Relative lengths of fingers and toes in human males and females |journal=Hormones and Behavior |volume=42 |issue=4 |pages=492–500 |year=2002 |month=December |pmid=12488115 |doi=10.1006/hbeh.2002.1833}}</ref><ref name = "Hall and Love (2003)">{{Cite journal|author=Hall LS, Love CT |title=Finger-length ratios in female monozygotic twins discordant for sexual orientation |journal=Archives of Sexual Behavior |volume=32 |issue=1 |pages=23–8 |year=2003 |month=February |pmid=12597269 |doi=10.1023/A:1021837211630}}</ref><ref name = "Rahman and Wilson (2003)">{{Cite journal|author=Rahman Q, Wilson GD |title=Sexual orientation and the 2nd to 4th finger length ratio: evidence for organising effects of sex hormones or developmental instability? |journal=Psychoneuroendocrinology |volume=28 |issue=3 |pages=288–303 |year=2003 |month=April |pmid=12573297 |doi=10.1016/S0306-4530(02)00022-7}}</ref><ref name = "Putz, D.A. et al. (2004)">{{Cite journal|doi=10.1016/j.evolhumbehav.2004.03.005 |title=Sex hormones and finger length: What does 2D:4D indicate? |month=May |year=2004 |first1=David A. |last1=Putz |first2=Steven J. C. |last2=Gaulin |first3=Robert J. |last3=Sporter |first4=Donald H. |last4=McBurney |journal=Evolution and Human Behavior |volume=25 |issue=3 |pages=182–99 |url=http://www.anth.ucsb.edu/faculty/gaulin/page1/Puts_et_al_2004.pdf}}</ref><ref name = "Rahman, Q. (2005)">{{Cite journal|author=Rahman Q |title=Fluctuating asymmetry, second to fourth finger length ratios and human sexual orientation |journal=Psychoneuroendocrinology |volume=30 |issue=4 |pages=382–91 |year=2005 |month=May |pmid=15694118 |doi=10.1016/j.psyneuen.2004.10.006}}</ref><ref name = "Kraemer et al. (2006)">{{Cite journal|author=Kraemer B, Noll T, Delsignore A, Milos G, Schnyder U, Hepp U |title=Finger length ratio (2D:4D) and dimensions of sexual orientation |journal=Neuropsychobiology |volume=53 |issue=4 |pages=210–4 |year=2006 |pmid=16874008 |doi=10.1159/000094730}}</ref><ref name = "Wallien et al. (2008)">{{Cite journal|author=Wallien MS, Zucker KJ, Steensma TD, Cohen-Kettenis PT |title=2D:4D finger-length ratios in children and adults with gender identity disorder |journal=Hormones and Behavior |volume=54 |issue=3 |pages=450–4 |year=2008 |month=August |pmid=18585715 |doi=10.1016/j.yhbeh.2008.05.002}}</ref><ref name = "Grimbos et al. (2010)">{{Cite journal|author=Grimbos T, Dawood K, Burriss RP, Zucker KJ, Puts DA |title=Sexual orientation and the second to fourth finger length ratio: a meta-analysis in men and women |journal=Behav Neurosci |volume=124 |issue=2 |pages=278–287 |year=2010 |pmid=20364887 |doi=10.1037/a0018764 }}</ref> butch vs. femme lesbians.<ref name="Tortorice, J.L. (2002)"/><ref>{{Cite journal|author=Brown WM, Finn CJ, Cooke BM, Breedlove SM |title=Differences in finger length ratios between self-identified 'butch' and 'femme' lesbians |journal=Archives of Sexual Behavior |volume=31 |issue=1 |pages=123–7 |year=2002 |month=February |pmid=11910785 |doi=10.1023/A:1014091420590 |url=https://www.msu.edu/~breedsm/pdf/ButchFemme.pdf}}</ref>
* Gay vs straight men,<ref name = "McFadden and Shubel (2002)"/> but most studies find very little difference in digit ratio between gay and straight men. One study found that Gay men, with several older brothers tend to have different digit arrangements.<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/sci/tech/695142.stm/ | deadurl=yes}} {{Dead link|date=June 2012|bot=RjwilmsiBot}}</ref><ref name = "Grimbos et al. (2010)"/> Some studies correlate male homosexuality and 2D:4D positively,<ref>{{cite journal | author = Churchchill AJG, Manning JT, Peters M | year = 2007 | title = The effects of sex, ethnicity, and sexual orientation on self-measured digit ratio (2D:4D) | url = | journal = Archives of Sexual Behavior | volume = 36 | issue = 2| pages = 251–260 | doi=10.1007/s10508-006-9166-8 | pmid=17394056}}</ref> others negatively.<ref>{{Cite journal|author=S.J. Robinson, J.T. Manning |journal=Evolution and Human Behavior |title=The ratio of 2nd to 4th digit length and male homosexuality |volume=21 |year=2000 |pages=333–345 |doi=10.1016/S1090-5138(00)00052-0 |pmid=11053694 |issue=5}}</ref>


==Cognitive and physical traits==
* Difference in digit ratio between identical female twins discordant for sexual orientation.<ref name = "Hall and Love (2003)"/>
The 2D:4D ratio has been associated with a variety of traits including academic ability, personality as well as physical attributes such as health outcomes. Lower (masculinized) 2D:4D ratios have been associated with higher mathematical ability as well as personality traits such as aggressiveness (in males), assertiveness (in females) and reduced [[agreeableness]].<ref name="Jeevanandam_2016" /><ref>{{cite journal | vauthors = Luxen MF, Buunk BP | title = Second-to-fourth digit ratio related to verbal and numerical intelligence and the Big Five | journal = Personality and Individual Differences | volume = 39 | issue = 5 | pages = 959–966 | date = 2005 | pmid = | doi = 10.1016/j.paid.2005.03.016| s2cid = }}</ref><ref>{{cite journal | vauthors = Hampson E, Sankar JS | title = Re-examining the Manning hypothesis: androgen receptor polymorphism and the 2D: 4D digit ratio. | journal = Evolution and Human Behavior | volume = 33 | issue = 5 | pages = 557–561 | date = 2012 | pmid = | doi = 10.1016/j.evolhumbehav.2012.02.003| bibcode = 2012EHumB..33..557H | s2cid = }}</ref> Males with higher (feminized) ratios are reported to be more at risk of low sperm counts, heart disease, obesity, [[metabolic syndrome]] and depression than those with lower ratios.<ref>{{cite journal | vauthors = Oyeyemi BF, Iyiola OA, Oyeyemi AW, Oricha KA, Anifowoshe AT, Alamukii NA| title = Sexual dimorphism in ratio of second and fourth digits and its relationship with metabolic syndrome indices and cardiovascular risk factors. | journal = Journal of Research in Medical Sciences | volume = 19 | issue = 3 | pages = 234–239 | date = 2014 | pmid = 24949031| doi = | pmc = 4061645 | s2cid = }}</ref><ref>{{cite journal | vauthors = Bailey Allison A, Hurd PL | title = Depression in men is associated with more feminine finger length ratios. | journal = Personality and Individual Differences | volume = 39 | issue = 4 | pages = 829–836 | date = 2005 | pmid = | doi = 10.1016/j.paid.2004.12.017| s2cid = }}</ref><ref>{{cite journal | vauthors = Coyne SM, Manning JT, Ringer L, Bailey L | title = Directional asymmetry (right–left differences) in digit ratio (2D: 4D) predict indirect aggression in women. | journal = Personality and Individual Differences | volume = 43 | issue = 4 | pages = 865–872 | date = 2007 | pmid = | doi = 10.1016/j.paid.2007.02.010| s2cid = }}</ref> Females with higher ratios are more at risk of eating disorders, anxiety and depression.<ref>{{cite journal | vauthors = Manning JT, Kilduff LP, Trivers R | title = Digit ratio (2D: 4D) in Klinefelter's syndrome | journal = Andrology | volume = 1 | issue = 1 | pages = 94–99 | date = 2013 | pmid = 23258636| doi = 10.1111/j.2047-2927.2012.00013.x| s2cid = }}</ref>
* [[Fraternal birth order and sexual orientation|Fraternal birth order effect]] on digit ratio.<ref name = "Williams, T.J. et al. (2000)"/>
* [[Paraphilia|Paraphilic]] (or extreme fetish) sexual interest.<ref>{{cite doi|10.1007/s10508-007-9255-3}}</ref> (see also: [[List of paraphilias]])


Prenatal androgen exposure has been implicated as a determinant in left-handedness, though contradictory and inconclusive findings have been reported concerning an association between 2D:4D and left-hand dominance.<ref>{{cite journal | vauthors = Trabert B, Graubard BI, Erickson RL, Zhang Y, McGlynn KA | title = Second to fourth digit ratio, handedness and testicular germ cell tumors. | journal = Early Human Development | volume = 89 | issue = 7 | pages = 463–466 | date = 2013 | pmid = 23623693| doi = 10.1016/j.earlhumdev.2013.04.001| pmc = 3684556 | s2cid = }}</ref><ref>{{cite journal | vauthors = Lust JM, Geuze RH, Van de Beek C, Cohen-Kettenis PT, Bouma A, Groothuis TG | title = Differential effects of prenatal testosterone on lateralization of handedness and language. | journal = Neuropsychology | volume = 25 | issue = 5| pages = 581–589 | date = 2011 | pmid = 21500918| doi = 10.1037/a0023293| s2cid = }}</ref><ref>{{cite journal | vauthors = Manning JT, Peters M | title = Digit ratio (2D:4D) and hand preference for writing in the BBC Internet Study. | journal = Laterality | volume = 14 | issue = 5| pages = 528–540 | date = 2009 | pmid = 19177258| doi = 10.1080/13576500802637872| s2cid = }}</ref> Kumar ''et al.'' have reported that hand preference is most strongly associated with 3D:4D digit ratios and that the effect is reversed between children and adults.<ref>{{cite journal | vauthors = Kumar S, Singh M, Voracek M | title = Effects of hand preference on digit lengths and digit ratios among children and adults | journal = Early Human Development | volume = 151 | pages = 105204 | date = December 2020 | pmid = 33059164 | doi = 10.1016/j.earlhumdev.2020.105204 | s2cid = 222829822 | url = https://epublications.marquette.edu/math_fac/56 | access-date = 23 March 2022 | archive-date = 17 May 2022 | archive-url = https://web.archive.org/web/20220517050256/https://epublications.marquette.edu/math_fac/56/ | url-status = live }}</ref>
{{See also|Biology and sexual orientation|Neuroscience and sexual orientation#2D:4D digit ratio}}


Fink ''et al.'' (2004) found that men with low 2D:4D ratios (indicating high testosterone) and women with high 2D:4D ratios (indicating high estrogen) express greater levels of [[facial symmetry]].<ref>{{Cite journal | vauthors = Fink B, Manning JT, Neave N, Grammer K |doi=10.1016/S1090-5138(03)00084-9 |title=Second to fourth digit ratio and facial asymmetry |date=March 2004 |journal=Evolution and Human Behavior |volume=25 |issue=2 |pages=125–32|bibcode=2004EHumB..25..125F }}</ref> The prenatal sex hormone ratio (indicated by 2D:4D) was associated with similar alterations to both male and female face shapes in utero, though the effect was three times more pronounced in males than in females.<ref>{{cite journal | vauthors = Fink B, Grammer K, Mitteroecker P, Gunz P, Schaefer K, Bookstein FL, Manning JT | title = Second to fourth digit ratio and face shape | journal = Proceedings. Biological Sciences | volume = 272 | issue = 1576 | pages = 1995–2001 | date = October 2005 | pmid = 16191608 | pmc = 1559906 | doi = 10.1098/rspb.2005.3179 }}</ref>
===Transsexualism===
* A recent study in Germany has found a correlation between digit ratio and male to female [[transsexualism]]. Male to female transsexuals ([[trans women]]) were found to have a higher digit ratio than control males, but one that was comparable to control females.<ref>{{Cite journal|author=Schneider HJ, Pickel J, Stalla GK |title=Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure |journal=Psychoneuroendocrinology |volume=31 |issue=2 |pages=265–9 |year=2006 |month=February |pmid=16140461 |doi=10.1016/j.psyneuen.2005.07.005}}</ref>


In a non-clinical sample of women, digit ratio correlated with [[anogenital distance]] in the expected direction. In other words, women with a greater anogenital distance, indicating greater prenatal androgen exposure, had a smaller digit ratio.<ref name="Barrett_2015">{{cite journal | vauthors = Barrett ES, Parlett LE, Swan SH | title = Stability of proposed biomarkers of prenatal androgen exposure over the menstrual cycle | journal = Journal of Developmental Origins of Health and Disease | volume = 6 | issue = 2 | pages = 149–157 | date = April 2015 | pmid = 25584807 | pmc = 5119464 | doi = 10.1017/S2040174414000646 }}</ref> Ronalds ''et al.'' (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.<ref>{{cite journal | vauthors = Ronalds G, Phillips DI, Godfrey KM, Manning JT | title = The ratio of second to fourth digit lengths: a marker of impaired fetal growth? | journal = Early Human Development | volume = 68 | issue = 1 | pages = 21–26 | date = June 2002 | pmid = 12191526 | doi = 10.1016/s0378-3782(02)00009-9 }}</ref>
===Digit ratio and development===
There is some evidence that 2D:4D ratio may also be indicative for human development and growth. Ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.{{Citation needed|reason=Ronalds 2002 is not in the list of references|date=October 2009}} Moreover, studies about 2D:4D correlations with face shape suggest that testosterone exposure early in life may set some constraints for subsequent development. Prenatal sex steroid ratios (in terms of 2D:4D) and actual chromosomal sex dimorphism were found to operate differently on human faces, but affect male and female face shape by similar patterns.<ref>{{Cite journal|author=Fink B |title=Second to fourth digit ratio and face shape |journal=Proceedings. Biological Sciences |volume=272 |issue=1576 |pages=1995–2001 |year=2005 |month=October |pmid=16191608 |pmc=1559906 |doi=10.1098/rspb.2005.3179|author-separator=,|author2=Grammer K|author3=Mitteroecker P|display-authors=3|last4=Gunz|first4=P.|last5=Schaefer|first5=K.|last6=Bookstein|first6=F. L|last7=Manning|first7=J. T}}</ref> However, exposure to very high levels of testosterone and/or estrogen in the womb may have negative effects as well. Fink et al. (2004) found that men with low (indicating high testosterone) and women with high (indicating high estrogen) 2D:4D ratios express greater levels of [[facial symmetry]].<ref>{{Cite journal|doi=10.1016/S1090-5138(03)00084-9 |title=Second to fourth digit ratio and facial asymmetry |month=March |year=2004 |first1=Bernhard |last1=Fink |first2=John T. |last2=Manning |first3=Nick |last3=Neave |first4=Karl |last4=Grammer |journal=Evolution and Human Behavior |volume=25 |issue=2 |pages=125–32}}</ref>


In 2020, a paper by John T Manning and Bernhard Fink reported that at the national level, high mean male 2D:4D was associated with high [[case fatality rate]]s with [[COVID-19]], and suggested that this may be used to "identify for whom it would be advisable to exercise [[social distancing]]."<ref name="Manning_2020">{{cite journal | vauthors = Manning JT, Fink B | title = Understanding COVID-19: Digit ratio (2D:4D) and sex differences in national case fatality rates | journal = Early Human Development | volume = 146 | pages = 105074 | date = July 2020 | pmid = 32419720 | pmc = 7224643 | doi = 10.1016/j.earlhumdev.2020.105074 }}</ref> Subsequent researchers failed to replicate their findings,<ref>{{cite journal | vauthors = Jones AL, Jaeger B, Schild C | title = No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities | journal = Early Human Development | volume = 152 | pages = 105272 | date = January 2021 | pmid = 33227636 | pmc = 7670914 | doi = 10.1016/j.earlhumdev.2020.105272 }}</ref><ref name="Jones_2020">{{cite journal | vauthors = Jones AL, Satchell LP, Jaeger B, Schild C | title = (Mis-)understanding COVID-19 and digit ratio: Methodological and statistical issues in Manning and Fink (2020) | journal = Early Human Development | volume = 148 | pages = 105095 | date = September 2020 | pmid = 32499091 | pmc = 7256504 | doi = 10.1016/j.earlhumdev.2020.105095 }}</ref> and criticised Manning and Fink for publishing papers that fail under scrutiny despite the urgent need for high quality science during the pandemic that informs medical decisions.<ref name="Jones_2020" />
==Digit ratio and palaeolithic hand stencils==
2D:4D is being used alongside other methods to help sex Palaeolithic hand stencils found in European and Indonesian caves.<ref>{{Cite journal|first1=Dean R. |last1=Snow |year=2006 |title=Sexual dimorphism in Upper Palaeolithic hand stencils |url=http://antiquity.ac.uk/ant/080/ant0800390.htm |journal=Antiquity |volume=80 |issue=308 |pages=390–404}}</ref><ref name="bradshawfoundation.com">{{Cite journal|first1=Jean-Michel |last1=Chazine |first2=Arnaud |last2=Noury |year=2006 |title=Sexual Determination of Hand Stencils at the Masri II Cave |url=http://www.bradshawfoundation.com/inora/divers_44_1.html |journal=Inora Newsletter |volume=44 |pages=21–6}}</ref><ref name="waspjournals.com">{{Cite journal|first1=Emma C. |last1=Nelson |first2=John T. |last2=Manning |first3=Anthony G. M. |last3=Sinclair |year=2006 |title=Using the length of the 2nd to 4th digit ratio (2D:4D) to sex cave art hand stencils: factors to consider |url=http://www.waspjournals.com/journals/beforefarming/journal_20061/news/2006_1_06.pdf |journal=Before Farming |volume=1 |issue=6 |pages=1–7}}</ref>


==Explanation of the digit ratio effect==
==Digit ratio research in animals==
The mechanisms by which prenatal hormones influence digit ratio are not clearly understood. Traits such as [[otoacoustic emissions]] and arm-to-trunk length ratio are also influenced by prenatal hormones, and it is postulated that [[Hox genes]] responsible for both digit and penis development play an important role in affecting these multiple traits ([[pleiotropy]]).<ref>{{cite journal | vauthors = Dickman S | title = HOX gene links limb, genital defects | journal = Science | volume = 275 | issue = 5306 | pages = 1568 | date = March 1997 | pmid = 9072822 | doi = 10.1126/science.275.5306.1568 | s2cid = 33709778 }}</ref> Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes.
* Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of [[great apes]], including gorillas and chimpanzees.<ref name = "McFadden and Shubel (2002)"/>
* Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.<ref>{{cite web|first=Emma |last=Nelson |title=Investigating relationships between the second-to-fourth digit ratio (2D:4D), social and bonding behaviours in non-human anthropoids |url=https://sites.google.com/site/enelson67profile/Home/profile/research/current-research |accessdate=29 October 2009}}{{dead link|date=October 2011}}{{Self-published inline|date=October 2009}}</ref>
* Sexual dimorphism in hind limb 2D:4D has been demonstrated in mice by two studies by both John Manning and [[Marc Breedlove]]'s research groups. There is some evidence to suggest that this effect is not seen in all mouse strains.{{Citation needed|date=October 2009}}
* Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.{{Citation needed|date=October 2009}}
* Front limb D2:D3 has shown to be influenced by prenatal alcohol exposure in female rats.{{Citation needed|date=October 2009}}
* Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.<ref>{{Cite journal|author=Talarovičová A, Kršková L, Blažeková J |title=Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood |journal=Hormones and Behavior |volume=55 |issue=1 |pages=235–9 |year=2009 |month=January |pmid=19022257 |doi=10.1016/j.yhbeh.2008.10.010}}</ref>
* [[Peter L. Hurd]], [[Theodore Garland, Jr.]], and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see [[experimental evolution]]). These high-runner mice exhibit increased 2D:4D. This apparent "feminization" is opposite to the relation seen between 2D:4D and physical fitness in human beings, and is difficult to reconcile with the idea that 2D:4D is a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect effect of glucocorticoids or other factors that regulate any of various genes.<ref>{{Cite journal|author=Yan RH, Malisch JL, Hannon RM, Hurd PL, Garland T |title=Selective Breeding for a Behavioral Trait Changes Digit Ratio |journal=PLoS ONE |volume=3 |issue=9 |pages=e3216 |year=2008 |pmid=18797502 |pmc=2528935 |doi=10.1371/journal.pone.0003216|editor1-last=Svensson|editor1-first=Erik I.}}</ref>


Though it is unclear why the digit ratio on the right hand should be more sensitive to prenatal hormones than that on the left hand, as indicated by the greater sex difference on the right than the left,<ref name="Honekopp_2010">{{cite journal | vauthors = Hönekopp J, Watson S | title = Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand | journal = American Journal of Human Biology | volume = 22 | issue = 5 | pages = 619–630 | year = 2010 | pmid = 20737609 | doi = 10.1002/ajhb.21054 | s2cid = 29503255 }}</ref> these differences are not evident in dorsal measurements<ref>{{cite journal | vauthors = Kumar S, Voracek M, Singh M | title = The effects of hand preference and sex on right-left asymmetry in dorsal digit lengths among adults and children | journal = Early Human Development | volume = 153 | pages = 105293 | date = February 2021 | pmid = 33340946 | doi = 10.1016/j.earlhumdev.2020.105293 | s2cid = 229333079 }}</ref> nor bone digit ratios,<ref>{{cite journal | vauthors = Xi H, Li M, Fan Y, Zhao L | title = A comparison of measurement methods and sexual dimorphism for digit ratio (2D:4D) in Han ethnicity | journal = Archives of Sexual Behavior | volume = 43 | issue = 2 | pages = 329–333 | date = February 2014 | pmid = 24013635 | pmc = 3890058 | doi = 10.1007/s10508-013-0179-9 }}</ref><ref name="Galis_2010"/><ref>{{cite journal | vauthors = Robertson J, Zhang W, Liu JJ, Muir KR, Maciewicz RA, Doherty M | title = Radiographic assessment of the index to ring finger ratio (2D:4D) in adults | journal = Journal of Anatomy | volume = 212 | issue = 1 | pages = 42–48 | date = January 2008 | pmid = 18005121 | pmc = 2423386 | doi = 10.1111/j.1469-7580.2007.00830.x }}</ref> and it is possible that right-left differences in palmar 2D:4D may be attributed to differential placing of flexion creases.<ref name="Kumar_2019" />
==See also==
* [[Anogenital distance]]
* [[Waist–hip ratio]]
* [[Dermatoglyphics]]
* [[Body mass index]]
* [[Handedness and sexual orientation]]
* [[Chiromancy]] (Pseudoscientific Hand analysis)


==Notes==
==Criticism==
The credibility of research into the relationship between digit ratios and traits has been subject to criticism for various reasons.
{{Reflist|2}}


# Weaknesses in the statistical significance of numerous findings.<ref name="Voracek_2019">{{cite journal | vauthors = Voracek M, Kossmeier M, Tran US | title=Which Data to Meta-Analyze, and How? | journal=Zeitschrift für Psychologie | publisher=Hogrefe Publishing Group | volume=227 | issue=1 | year=2019 | issn=2190-8370 | doi=10.1027/2151-2604/a000357 | pages=64–82| s2cid=86788835 }}</ref><ref name="Jones_2021">{{cite journal | vauthors = Jones AL, Jaeger B, Schild C | title = No credible evidence for links between 2D:4D and COVID-19 outcomes: A probabilistic perspective on digit ratio, ACE variants, and national case fatalities | journal = Early Human Development | volume = 152 | pages = 105272 | date = January 2021 | pmid = 33227636 | pmc = 7670914 | doi = 10.1016/j.earlhumdev.2020.105272 | publisher = Elsevier BV }}</ref><ref name="Voracek Pietschnig Nader Stieger 2011 pp. 417–422">{{cite journal | vauthors = Voracek M, Pietschnig J, Nader IW, Stieger S | title=Digit ratio (2D:4D) and sex-role orientation: Further evidence and meta-analysis | journal=Personality and Individual Differences | publisher=Elsevier BV | volume=51 | issue=4 | date= September 2011 | issn=0191-8869 | doi=10.1016/j.paid.2010.06.009 | pages=417–422}}</ref>
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# Irreproducibility of findings in many instances, with some studies reporting contradictory results.<ref name="Hilgard_2019">{{cite journal | vauthors = Hilgard J, Engelhardt CR, Rouder JN, Segert IL, Bartholow BD | title = Null Effects of Game Violence, Game Difficulty, and 2D:4D Digit Ratio on Aggressive Behavior | journal = Psychological Science | volume = 30 | issue = 4 | pages = 606–616 | date = April 2019 | pmid = 30843758 | doi = 10.1177/0956797619829688 | publisher = SAGE Publications | s2cid = 73496727 }}</ref><ref name="Leslie_2019">{{cite journal | url=https://www.science.org/news/2019/06/talk-hand-scientists-try-debunk-idea-finger-length-can-reveal-personality-and-health | vauthors=Leslie M | title=Talk to the hand. Scientists try to debunk idea that finger length can reveal personality and health | journal=Science | year=2019 | doi=10.1126/science.aay2735 | s2cid=241187298 | access-date=1 October 2021 | archive-date=10 October 2021 | archive-url=https://web.archive.org/web/20211010065849/https://www.science.org/news/2019/06/talk-hand-scientists-try-debunk-idea-finger-length-can-reveal-personality-and-health | url-status=live }}</ref><ref name="Medland_2010">{{cite journal | vauthors = Medland SE, Zayats T, Glaser B, Nyholt DR, Gordon SD, Wright MJ, Montgomery GW, Campbell MJ, Henders AK, Timpson NJ, Peltonen L, Wolke D, Ring SM, Deloukas P, Martin NG, Smith GD, Evans DM | display-authors = 6 | title = A variant in LIN28B is associated with 2D:4D finger-length ratio, a putative retrospective biomarker of prenatal testosterone exposure | journal = American Journal of Human Genetics | volume = 86 | issue = 4 | pages = 519–525 | date = April 2010 | pmid = 20303062 | pmc = 2850436 | doi = 10.1016/j.ajhg.2010.02.017 }}</ref><ref name="Wong_2016"/><ref name="Pratt_2016">{{cite journal | vauthors = Pratt TC, Turanovic JJ, Cullen FT | title=Revisiting the Criminological Consequences of Exposure to Fetal Testosterone: A Meta-Analysis of the 2D:4D Digit Ratio | journal=Criminology | publisher=Wiley | volume=54 | issue=4 | date=22 September 2016 | issn=0011-1384 | doi=10.1111/1745-9125.12115 | pages=587–620}}</ref>
# Lack of a clear [[Correlation does not imply causation|cause-and-effect]] relationship between digit ratio and traits.<ref name="McCormick_2020">{{cite journal | vauthors = McCormick CM, Carré JM | title = Facing off with the phalangeal phenomenon and editorial policies: A commentary on Swift-Gallant, Johnson, Di Rita and Breedlove (2020) | journal = Hormones and Behavior | volume = 120 | pages = 104710 | date = April 2020 | pmid = 32057822 | doi = 10.1016/j.yhbeh.2020.104710 | publisher = Elsevier BV | s2cid = 211112860 }}</ref><ref name="Jones_2021"/><ref name="Smoliga_2021" />


Concerning the evidence for causal relationships, the reliability of digit ratio as a [[proxy variable]] has come under scrutiny.<ref name="Leslie_2019">{{cite journal | url=https://www.science.org/news/2019/06/talk-hand-scientists-try-debunk-idea-finger-length-can-reveal-personality-and-health | vauthors=Leslie M | title=Talk to the hand. Scientists try to debunk idea that finger length can reveal personality and health | journal=Science | year=2019 | doi=10.1126/science.aay2735 | s2cid=241187298 | access-date=1 October 2021 | archive-date=10 October 2021 | archive-url=https://web.archive.org/web/20211010065849/https://www.science.org/news/2019/06/talk-hand-scientists-try-debunk-idea-finger-length-can-reveal-personality-and-health | url-status=live }}</ref><ref name="McCormick_2020"/><ref name="Voracek_2019" /> Some studies that investigate the relationship between a trait and an unobservable variable fail to consider possible [[confounding variables]].<ref>{{cite journal | vauthors = Lolli L, Batterham AM, Kratochvíl L, Flegr J, Weston KL, Atkinson G | title = A comprehensive allometric analysis of 2nd digit length to 4th digit length in humans | journal = Proceedings. Biological Sciences | volume = 284 | issue = 1857 | pages = 20170356 | date = June 2017 | pmid = 28659446 | pmc = 5489719 | doi = 10.1098/rspb.2017.0356 }}</ref> This is especially problematic in cases where the relationship is likely to be weak, such as with prenatal testosterone.<ref name="Hollier_2015">{{cite journal | vauthors = Hollier LP, Keelan JA, Jamnadass ES, Maybery MT, Hickey M, Whitehouse AJ | title = Adult digit ratio (2D:4D) is not related to umbilical cord androgen or estrogen concentrations, their ratios or net bioactivity | journal = Early Human Development | volume = 91 | issue = 2 | pages = 111–117 | date = February 2015 | pmid = 25594498 | doi = 10.1016/j.earlhumdev.2014.12.011 | publisher = Elsevier BV | hdl = 20.500.11937/48405 | url = https://research-repository.uwa.edu.au/en/publications/b3d44281-0fcf-499c-9743-7daac128c486 }}</ref><ref name="Wong_2016">{{cite journal | vauthors = Wong WI, Hines M | title = Interpreting digit ratio (2D:4D)-behavior correlations: 2D:4D sex difference, stability, and behavioral correlates and their replicability in young children | journal = Hormones and Behavior | volume = 78 | pages = 86–94 | date = February 2016 | pmid = 26542674 | doi = 10.1016/j.yhbeh.2015.10.022 | publisher = Elsevier BV | s2cid = 26592548 }}</ref><ref name="Voracek_2019" /> Furthermore, critics have pointed out that controlling for confounding variables is an impossible task due to the expanding list of confounding variables, including ethnicity, sex,<ref name="Manning_2004" /><ref name="Lindová_2008">{{cite journal | vauthors = Lindová J, Hrušková M, Pivoňková V, Kuběna A, Flegr J | title=Digit ratio (2D:4D) and Cattell's personality traits | journal=European Journal of Personality | publisher=SAGE Publications | volume=22 | issue=4 | date = June 2008 | issn=0890-2070 | doi=10.1002/per.664 | pages=347–356| s2cid=18205251 }}</ref> and physical, medical or behavioral conditions.<ref name="McQuade_2011">{{cite journal | vauthors = McQuade DB | title = Does digit ratio (2D:4D) predict penile length? | journal = Asian Journal of Andrology | volume = 13 | issue = 5 | pages = 667–668 | date = September 2011 | pmid = 21725331 | pmc = 3739583 | doi = 10.1038/aja.2011.81 | publisher = Medknow }}</ref>
==References==
{{Refbegin}}
//Listed above under PMID 15620791 with the correct title//* Bailey AA & [[Peter L. Hurd|Hurd PL]], 2005. Finger length ratio predicts physical aggression in men but not women. ''Biological Psychology'' '''68''': 215-222
//Listed above under PMID 19819951 with complete publication info//*Berenbaum SA, Korman Bryk K, Nowak N, Quigley CA, Moffat S. (2009) Fingers as a Marker of Prenatal Androgen Exposure. Endocrinology. 2009 Oct 9. [Epub ahead of print] PMID 19819951.[http://www.ncbi.nlm.nih.gov/pubmed/19819951?ordinalpos=2&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum]
//Listed above under PMID 11910785 with correct title//* Brown, W. M., Finn, C. J., Cooke, B. M., & Breedlove, S. M. (2002). [http://msu.edu/~breedsm/pdf/ButchFemme.pdf Differences in finger length between self-identified "butch" and "femme" lesbians.] ''Archives of Sexual Behavior, 31,'' 123-128.
//Listed above under PMID 12488105//*Brown WM, Hines M, Fane BA, Breedlove SM. (2002)Masculinized finger length patterns in human males and females with congenital adrenal hyperplasia. Horm Behav. 2002 Dec;42(4):380-6 [http://www.ncbi.nlm.nih.gov/pubmed/12488105?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum].
//Listed above under PMID 17203400 with correct title and DOI//* Burriss, R.P., Little, A.C. and Nelson, E.C. 2006. 2D:4D ratio and its relationship with sexually dimorphic facial characteristics and perceived dominance. Archives of Sexual Behavior. DOI: 1007/s10508-006-9136-1 [http://www.digitratio.com/2D4DFaceshape.pdf 2D:4D and sexually dimorphic facial characteristics and perceived dominance]
//Listed above with URL http://www.bradshawfoundation.com/inora/divers_44_1.html//* Chazine, J-M. and Noury, A. 2006. Sexual Determination of hand stencils on the main panel of the Gua Masri II cave (East-Kalimantan/Borneo - Indonesia). International Newsletter On Rock Art (INORA), 44: 21-26.
//Listed above under PMID 19139402//* {{cite journal| title=Second-to-fourth digit ratio predicts success among high-frequency financial traders| author=Coates JM, Gurnell M, Rustichini A. |journal=Proc Natl Acad Sci U S A. |date=12-01-2009 |pmid=19139402| volume=106| issue=2| pages=623–8| doi=10.1073/pnas.0810907106| pmc=2626753}}
//Listed above under PMID 12581689//* Csathó, A., Osváth, A., Bicsák, E., Karádi, Manning, J., Kállai, J. (2003) Sex role identity related to the ratio of second to fourth digit length in women. ''Biological Psychology, 62,'' 147-156
!!UNCLEAR!!* Fink, B., Manning, J.T., Neave, N. & Tan, U. (2004). Second to fourth digit ratio and hand skill in Austrian children. ''Biological Psychology, 67(3),'' 375-384. [http://evolution.anthro.univie.ac.at/institutes/urbanethology/resources/pdf/2D4D-BIOLPSY-2004.pdf Full text]
!!UNCLEAR!!* Fink, B., Manning, J.T., Neave, N. & Grammer, K. (2004). Second to fourth digit ratio and facial asymmetry. ''Evolution and Human Behavior, 25(2),'' 125-132.
//Listed above under PMID 16261185//* Fink, B., Manning J.T., & Neave, N. (2005). The 2nd to 4th Digit Ratio and Neck Circumference: Implications for Risk Factors in Coronary Heart Disease. ''International Journal of Obesity.''
//Listed above under PMID 16191608//* Fink, B., Grammer, K., Mitteroecker, P., Gunz, P., Schaefer, K., Bookstein, F.L. & Manning J.T. (2005). Second to fourth digit ratio and face shape. Proceedings of the Royal Society of London B: Biological Sciences, 272, 1995-2001.
//Listed above under PMID 12597269//* Hall LS, Love CT. (2003) Finger-length ratios in female monozygotic twins discordant for sexual orientation. Arch Sex Behav. 2003 Feb;32(1):23-8.
//Listed above under PMID 15113628//* Lutchmaya, S., Baron-Cohen, S., Raggatt, P., Knickmeyer, R. & Manning, J.T. (2004). '2nd to 4th digit ratios, fetal testosterone and estradiol' Early Human Development 77, 23-28.
//Listed above under PMID 9853845 with correct title//* Manning, J.T., Wilson, D.J. and Lewis-Jones, D.I. (1998). The ration of 2nd to 4th digit length: a predictor of sperm numbers and concentration to testosterone, leteinizing hormone and oestrogen, Human Reproduction, 13 (11), 3000-3004
//Listed above under PMID 10828555//* Manning, J.T., Barley, L., Walton, J., Lewis-Jones, D.I., [[Robert Trivers|Trivers, R.L.]], Singh, D., Thornhill, R., Rohde, P., Bereckei, T., Henzi, P., Soler, M. & Sved, A. (2000). The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success: evidence for sexually antagonistic genes. Evolution and Human Behavior. 21, 163-183
//Listed above under ISBN 978-0-8135-3030-7//* Manning, J.T. (2002). Digit ratio: a pointer to fertility, behaviour, and health. Rutgers U. Press. New Brunswick, NJ.
//Listed above under PMID 19819951//* Manning, J.T., Bundred, P.E., Newton, D.J., & Flanigan, B.F. (2003). 'The second to fourth digit ratio and variation in the androgen receptor gene' Evolution and Human Behavior 24, 399-405.
!!??NOT USED??!!* Manning, J.T., Fink, B., Neave, N., & Caswell, N. (2005) 'Photocopies Yield Lower Digit Ratios (2D:4D) Than Direct Finger Measurements.' Archives of Sexual Behavior 34(3), 329-333.
//Listed above under PMID 15500996//* Manning, J.T., Stewart, A., Bundred, P.E. & [[Robert Trivers|Trivers, R.L.]] (2004). Sex and ethnic differences in 2nd to 4th digit ratio of children. Early Human Development 80, 161-168.
//Listed above under PMID 14627046//* McIntyre, M.H. (2003). 'Digit ratios, childhood gender role behavior and erotic role preferences of gay men' Archives of Sexual Behavior 32, 495-497.
//Listed above under PMID 16504142//* McIntyre, M.H. (2006). 'The use of digit ratios as markers for perinatal androgen action' Reproductive Biology and Endocrinology 4, 10. (http://www.rbej.com/content/4/1/10)
//Listed above under PMID 14561281 with corrected title//* Neave, N., Laing, S., Fink, B. & Manning, J.T. (2003). Second to fourth digit ratio, testosterone, and perceived male dominance. Proceedings of the Royal Society of London B: Biological Sciences, 270, 2167-2172
//Listed above with URL http://www.waspjournals.com/journals/beforefarming/journal_20061/news/2006_1_06.pdf//* Nelson, E.C., Manning, J.T. and Sinclair, A.G.M. 2006. Using the length of the 2nd to 4th digit ratio to sex cave art hand stencils: factors to consider. Before Farming 2006/1 article 7 [https://2823605089376246246-a-1802744773732722657-s-sites.googlegroups.com/site/enelson67profile/Home/profile/research/publications/Nelsonetal.2006.pdf?attredirects=0&auth=ANoY7crLxs_AvSzbTMQeMFsypJ6zZF6FgSfcBB9R4_1oFu4Fw6LiGkLaFQ109k9LP51qZe1xVwc0JhtTsymqVKwCWkL6QEZfNS9lQQ-E8IHLEcq6MhD0FX7PbxS559LRK34ZGwMuNIILYDOslNoVEI9KFzOIldO71EUnP1tXSMFOBcHJHacCyDiHd0SuxSsAmTZ9RZcPijKcqK53WSTIpk4-_i5SaZh8l_C9MnQujIJgTu9BR-Sclq5g-NGqp5E97SrvbsBVbR3zL_nmiJvq_NRlSLxJaSJupg%3D%3D]
!!CAN'T FIND THIS ANYWHERE EXCEPT LISTED ON THE AUTHOR'S WEBSITE: https://sites.google.com/site/enelson67profile/Home/profile/research/publications !!* Nelson, E.C. and Shultz, S. 2007. Using the length of the 2nd to 4th digit ratio (2D:4D) to investigate the influence of prenatal sex hormones on non-human primate mating systems and human social evolution. ''American Journal of Physical Anthropology'' 132:177.
//Listed above under PMID 12441204//*Okten A, Kalyoncu M, Yariş N. (2002) The ratio of second- and fourth-digit lengths and congenital adrenal hyperplasia due to 21-hydroxylase deficiency. Early Hum Dev. 2002 Dec;70(1-2):47-54 [http://www.ncbi.nlm.nih.gov/pubmed/12441204?ordinalpos=4&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum].
//Listed above under PMID 17008344//* Paul, S.N., Kato, B.S., Hunkin, J.L., Vivekanandan, S. & Spector, T.D. (2006). The Big Finger - The second to fourth digit ratio (2d:4d) is a predictor of sporting ability in females. British Journal of Sports Medicine (http://bjsm.bmjjournals.com/cgi/content/abstract/bjsm.2006.027193)
//Listed above under PMID 15978716//* {{cite journal | last = Romano | first = Maria | coauthors = Barbara Leoni, Nicola Saino | title = Examination marks of male university students positively correlate with finger length ratios (2D:4D) | journal = Biological Psychology | volume = 71 | issue = 2 | pages = 175–182 | month = February | year = 2006 | url = http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T4T-4GFV5WY-1&_user=10&_coverDate=02%2F28%2F2006&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=fa6feef79cd5a3abf035313517779a39 | doi = 10.1016/j.biopsycho.2005.03.006 | accessdate = 2007-07-31 | pmid = 15978716}}
//Liested above under PMID 16140461//* {{cite journal | last = Schneider | first = Harald J. | coauthors = Johanna Pickel, Günter K. Stalla | title = Typical female 2nd-4th finger length (2D:4D) ratios in male-to-female transsexuals-possible implications for prenatal androgen exposure | journal = International Society of Psychoneuroendocrinology | volume = 31 | issue = 2 | pages = 265–269 | publisher = Elsevier, Oxford, UK | year = 2006 | url = http://www.sciencedirect.com/science/article/B6TBX-4H16P9S-1/2/ae91dff18b1b99385054e3bf971d47f9 | issn = 0306-4530 PMID 16140461 | accessdate = 2007-02-25 | doi = 10.1016/j.psyneuen.2005.07.005 | pmid = 16140461}}
//Listed above under URL http://antiquity.ac.uk/ant/080/ant0800390.htm //* Snow, D.R. 2006. Sexual dimorphism in Upper Palaeolithic hand stencils. Antiquity, 80:390-404.
//Listed above under PMID 19022257//* Talarovičová, A., Kršková, L., & Blažeková, J. (2008) [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WGC-4TTHWG5-2&_user=1256629&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000024219&_version=1&_urlVersion=0&_userid=1256629&md5=645ede7beff457699a68d30d3d2d5938 Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood]. Hormones and Behaviour 55, 235-239
//Listed above under PMID 10761903//* Williams, T. J., Pepitone, M. E., Christensen, S. E., Cooke, B. M., Huberman, A. D., Breedlove, N. J., Breedlove, T. J., Jordan, C. L., & Breedlove, S. M. (2000). [http://msu.edu/~breedsm/pdf/breedlove2000.pdf Finger-length ratios and sexual orientation.] ''Nature'', 404, 455-456.
//Listed above under {{doi|10.1016/0191-8869(83)90061-2}} //* Wilson, G.D. (1983). Finger-length as an index of assertiveness in women. Personality and Individual Differences 4, 111-112.
//Listed above under PMID 18797502//* Yan, R. H., Malisch, J. L., Hannon, R. M., Hurd, P. L. & Garland, T., Jr. (2008). [http://biology.ucr.edu/people/faculty/Garland/Yan_et_al_2008_2D4D.pdf Selective breeding for a behavioral trait changes digit ratio.] PLoS ONE 3(9):e3216. {{doi|10.1371/journal.pone.0003216}}.
{{Refend}}
-->


A 2021 paper by James Smoliga, titled "Giving science the finger—is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study" followed similar research methodologies adopted by others in the field and came to conclude that low digit ratio is associated with good luck.<ref name="Smoliga_2021" /> This result is attributed to chance and presented as an illustration of the [[Replication crisis|reproducibility crisis]]. Smoliga would later go on to label the field as pseudoscience.<ref>{{Cite web |title=Faculty Spotlight: James M. Smoliga {{!}} Tufts University School of Medicine |url=https://medicine.tufts.edu/news-events/news/faculty-spotlight-james-m-smoliga |access-date=2023-12-25 |website=medicine.tufts.edu |archive-date=25 December 2023 |archive-url=https://web.archive.org/web/20231225091109/https://medicine.tufts.edu/news-events/news/faculty-spotlight-james-m-smoliga |url-status=live }}</ref> In a response, John T. Manning criticized the researchers for not controlling for ethnicity.<ref>{{Cite journal |last=Manning |first=John T. |date=11 June 2023 |title=The Big Finger Trumps the Lucky Jim Fallacy |url=https://www.bmj.com/content/375/bmj-2021-067849/rr |journal=The BMJ |language=en |access-date=11 June 2023 |archive-date=11 June 2023 |archive-url=https://web.archive.org/web/20230611013153/https://www.bmj.com/content/375/bmj-2021-067849/rr |url-status=live }}</ref>
==External links==
*[http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=%22digit%20ratio%22or%22digit%20ratios%22or%222D%3A4D%22 PubMed listing of papers on digit ratios]


Researcher Marta Kowal states: "One of the general limitations of studies on the digit ratios is the high number of degrees of freedom within the 2D:4D (the right, left, and right minus left 2D:4D). Running many analyses with different predictors [[Multiple comparisons problem|increases the chances of finding allegedly significant results.]]"<ref name="No relationship between the digit r"/>
*{{Cite journal|first=Michael E. |last=Mills |month=October |year=2002 |title=Review of ''Digit Ratio: A Pointer to Fertility, Behavior and Health'' by John T. Manning |url=http://human-nature.com/nibbs/02/manning.html |journal=Human Nature Review |volume=2 |pages=418–23}}

{{Use dmy dates|date=September 2010}}
==Other animals==
* Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of [[great apes]], including gorillas and chimpanzees.<ref name = "McFadden_2002"/>
* Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.<ref>{{cite book | vauthors = Shultz S, Opie C, Nelson E, Atkinson QD, Dunbar RI |chapter=Evolution of Primate Social Systems: Implications for Hominin Social Evolution |pages=317–405 |chapter-url={{Google books|VEakAgAAQBAJ|page=317|plainurl=yes}} | veditors = Dunbar RI, Gamble C, Gowlett JA |year=2014 |title=Lucy to Language: The Benchmark Papers |publisher=OUP Oxford |isbn=978-0-19-965259-4 }}</ref>
* Nancy Burley's research group has demonstrated sexual dimorphism in [[zebra finch]]es, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.<ref>{{cite journal | vauthors = Burley NT | title = An eye for detail: selective sexual imprinting in zebra finches | journal = Evolution; International Journal of Organic Evolution | volume = 60 | issue = 5 | pages = 1076–1085 | date = May 2006 | pmid = 16817546 | doi = 10.1554/05-399.1 | s2cid = 28869542 | jstor = 4095408 }}</ref>
* Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.<ref>{{cite journal | vauthors = Talarovicová A, Krsková L, Blazeková J | title = Testosterone enhancement during pregnancy influences the 2D:4D ratio and open field motor activity of rat siblings in adulthood | journal = Hormones and Behavior | volume = 55 | issue = 1 | pages = 235–239 | date = January 2009 | pmid = 19022257 | doi = 10.1016/j.yhbeh.2008.10.010 | s2cid = 23727819 }}</ref>
* [[Peter L. Hurd]], [[Theodore Garland Jr.]], and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see [[experimental evolution]]). These high-runner mice exhibit increased 2D:4D in contradiction to the correlation between digit ratio and physical fitness in human beings, suggesting that 2D:4D may not be a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect the effect of glucocorticoids or other factors that regulate any of various genes.<ref>{{cite journal | vauthors = Yan RH, Malisch JL, Hannon RM, Hurd PL, Garland T | title = Selective breeding for a behavioral trait changes digit ratio | journal = PLOS ONE | volume = 3 | issue = 9 | pages = e3216 | date = September 2008 | pmid = 18797502 | pmc = 2528935 | doi = 10.1371/journal.pone.0003216 | doi-access = free | bibcode = 2008PLoSO...3.3216Y }}</ref>
* In [[pheasant]]s, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the [[Egg (biology)|egg]].<ref name="Romano_2006">{{cite journal | vauthors = Romano M, Leoni B, Saino N | title = Examination marks of male university students positively correlate with finger length ratios (2D:4D) | journal = Biological Psychology | volume = 71 | issue = 2 | pages = 175–182 | date = February 2006 | pmid = 15978716 | doi = 10.1016/j.biopsycho.2005.03.006 | s2cid = 39777964 }}</ref>
* Studies in mice indicate that prenatal androgen influences 2D:4D primarily by promoting growth of the fourth digit.<ref name="Zheng_2011" />

== See also ==
* [[Anogenital distance]]
* [[Body mass index]]
* [[Chiromancy]] (palm reading)
* [[Dermatoglyphics]]
* [[Handedness and sexual orientation]]
* [[Waist–hip ratio]]

== References ==
{{Reflist}}

== External links ==
{{Commons category|Digit ratio}}
*[https://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=search&term=%22digit%20ratio%22or%22digit%20ratios%22or%222D%3A4D%22 PubMed listing of papers on digit ratios]
*{{Cite journal| vauthors = Mills ME |date=October 2002 |title=Review of ''Digit Ratio: A Pointer to Fertility, Behavior and Health'' by John T. Manning |url=http://human-nature.com/nibbs/02/manning.html |journal=Human Nature Review |volume=2 |pages=418–23}}


{{DEFAULTSORT:Digit Ratio}}
{{DEFAULTSORT:Digit Ratio}}
[[Category:Androgens]]
[[Category:Anthropometry]]
[[Category:Gender]]
[[Category:Sex differences in humans]]
[[Category:Fingers]]
[[Category:Fingers]]
[[Category:Ratios]]
[[Category:Ratios]]
[[Category:Testosterone]]
[[Category:Testosterone]]

[[de:Fingerlängenverhältnis]]
[[fa:نسبت انگشت دوم به انگشت چهارم]]
[[fr:Indice de Manning]]
[[he:יחס אצבעות]]
[[nl:Wijsvinger-ringvinger-index]]
[[zh:指长比]]

Latest revision as of 23:46, 19 December 2024

For the observation about the frequencies of leading digits of numbers, see Benford's law.

Hand with index finger being shorter than the ring finger, resulting in a small 2D:4D ratio.
Image shows the measurement procedure of dorsal digit length using vernier calipers. The hand is placed on the edge of a table so that fingers are rested on table while making an angle of 90 degrees to the palm.

The digit ratio is a pseudoscience involving the ratio taken of the lengths of different digits or fingers on a hand.

The most commonly studied digit ratio is that of the 2nd (index finger) and 4th (ring finger), also referred to as the 2D:4D ratio, measured on the palm side. It is proposed that the 2D:4D ratio indicates the degree to which an individual has been exposed to androgens during key stages of fetal development. A lower ratio has been associated with higher androgen exposure, which would be the physiological norm for males but may also occur in some exceptional circumstances in females. The latter include developmental disorders such as congenital adrenal hyperplasia.

The 2D:4D ratio has been postulated to correlate with a range of physical and cognitive traits in childhood and adulthood, including personality traits such as assertiveness in women, aggressiveness in men, and cognitive abilities such as numerical skills.[1] It has also been shown to vary considerably between racial groups[2] with males having, on average, lower 2D:4D ratio than females.[3]

Studies in this field have drawn criticism over questionable statistical significance and difficulties in reproducing their findings[4][5][6] as well as lack of high quality research protocols.[7]

Measurement

[edit]

The 2D:4D ratio is calculated by dividing the length of the index finger by the length of the ring finger of the same hand. Other digit ratios are also calculated similarly in the same hand.

The digit length is typically measured on the palmar (ventral, "palm-side") hand, from the midpoint of the bottom crease to the tip of the finger.[8] Measurement of the digits on the dorsal ("back-side") hand, from the tip of the finger to the proximal phalange-bone protrusion (which occurs when digits are bent at 90 degrees to the palm) has also recently gained acceptance.[9][10] A study has shown that, compared to the palmar digit ratio, the dorsal digit ratio is a better indicator of bone digit ratio.[10] Furthermore, the palmar digit ratio is affected by the differential positioning of flexion creases.[11]

It has been alleged that, because academics have accepted a variety of techniques and equipment (such as calipers, radiography and photocopy), researchers record multiple measurements and report only those which have significant findings,[7] a form of reporting bias.

History of research

[edit]

Various reports in the scientific literature of the late 1800s noted that a greater proportion of men have shorter index fingers than ring fingers as compared to women.[12] By 1930, statistically significant sex differences in digit ratio were established in a sample of 201 men and 109 women, [13] after which time the sex difference appears to have been largely forgotten or ignored.

In 1983, Glenn Wilson of King's College London published a study examining the correlation between assertiveness in women and their digit ratio, which found that women with a lower 2D:4D ratio reported greater assertiveness.[14] This was the first study to examine the correlation between digit ratio and a psychological trait within members of the same sex.[15] Wilson proposed that skeletal structure and personality were simultaneously affected by sex hormone levels in utero.[14]

In 1998, John T. Manning and colleagues reported the sex difference in digit ratios was present in two-year-old children[16] and further developed the idea that the index was a marker of prenatal sex hormones. Since then, research on the topic has burgeoned around the world.

2D:4D digit ratios have been used alongside other methods in an attempt to understand Palaeolithic hand stencils found in prehistoric European and Indonesian cave painting.[17][18][19]

Sexual dimorphism and age

[edit]

Studies measuring the 2D:4D of fetuses have found the ratio to be sexually dimorphic, being lower in males than females.[20][21]

However, when examining the extent of sexual dimorphism evident after birth, adults were found to demonstrate more dimorphism than children, who show the same extent of dimorphism as fetuses.[22] This has led the authors to suggest that digit ratio is also affected by postnatal development. Similarly, a study of people from Poland found statistically significant variation in 2D:4D between age groups (children, young adults and adults) for both males and females in the left hand, but only for women with the right hand.[23]

Sexual dimorphism is strongest in digit ratios involving digit 2 with palmar measurements.[24][25][26] However, when measured dorsally, digit ratios involving digit 5 showed strongest dimorphism, with women having shorter fifth digits on average.[9] Overall, dorsal digit ratios demonstrate stronger sexual dimorphism than palmar digit ratios,[9] including the palmar 2D:4D ratio that has been the basis of most study.[27] Moreover, compared to palmar digit ratio, dorsal digit ratio is a better indicator of bone digit ratio.[10] Thus, while most of the earlier research has focused on palmar 2D:4D ratio, study of other digit ratios is also promising.

A 2009 study in Biology Letters argues: "Sexual differences in 2D:4D are mainly caused by the shift along the common allometric line with non-zero intercept, which means 2D:4D necessarily decreases with increasing finger length, and the fact that men have longer fingers than women",[28] which the authors claim may be the basis for the sex differences in 2D:4D and/or any putative hormonal influence on the ratios.

There has been no reliable correlation reported between 2D:4D and sex hormone levels in adulthood.[29][4] Marta Kowal has suggested that testosterone levels are correlated with other factors (such as smoking, body weight and diet) that would impact the relationship between 2D:4D and an adult's testosterone level.[4]

Effect of prenatal hormones

[edit]

It has been proposed that 2D:4D ratio is affected by fetal exposure to sex hormones, in particular to testosterone and other androgens. Lower 2D:4D is found to correlate with higher prenatal androgen exposure.[30][29][31][32][33][34][35][36] Therefore digit ratio could be considered a proxy variable (indirect measure) for prenatal androgen exposure. Various studies suggest that 2D:4D is also influenced by prenatal estrogen exposure, and that it correlates negatively, not with prenatal testosterone alone, but with the testosterone-to-estrogen ratio.[37][8][20][38]

Prenatal hormone exposure can be measured via amniocentesis (usually performed between 14th and 20th weeks of pregnancy), maternal serum sampling and umbilical cord sampling at birth, the latter being a measure of exposure in late gestation. A 2024 meta-analysis has suggested that digit ratio may be related to amniotic fluid testosterone levels, but not umbilical cord levels, requiring further validation.[39]

A 2011 paper by Zhengui Zheng and Martin J. Cohn reports that "the 2D:4D ratio in mice is controlled by the balance of androgen to estrogen signaling during a narrow window of digit development".[30] The formation of the digits in humans, in utero, is thought to occur by 13 weeks, and the bone-to-bone ratio is consistent from this point into an individual's adulthood.[40] If, during this period, the fetus is exposed to androgens (levels of which are usually far higher in male than female fetuses) the growth rate of the 4th digit is increased. In a 2006 study, digit ratio analysis of opposite-sex dizygotic twins found that the females in these pairings were born with significantly lower 2D:4D ratio, postulated to occur from exposure to excess androgens from their brothers in utero (the hormone-transfer theory).[41] However, an attempt to replicate these findings with a larger sample of dizygotic twins (867 individuals) found no differences in the variance or co-variance of same-sex and opposite-sex pairings to support the theory, though it did confirm female 2D:4D to be significantly higher than male as expected.[42]

Researchers have raised concerns that, although the general trend points towards a correlation between digit ratio and early androgen exposure, many results have not been statistically significant.[43]

Various findings have also challenged the general trend. One study of 66 children that attempted to replicate the findings of a frequently cited paper on the topic[37] found no association between prenatal testosterone and estrogen levels and 2D:4D in childhood.[44] Another paper also found no relationship between 2D:4D ratios and umbilical cord androgen and estrogen levels.[45] A large meta-analysis studying genomic correlations was unable to find evidence for 2D:4D being a marker for prenatal androgen exposure, but did not exclude the possibility given constraints in genomic knowledge (with 3.8% of the variance in 2D:4D ratio accounted for genetically).[46]

Developmental disorders

[edit]

Women with congenital adrenal hyperplasia (CAH), who have elevated androgen levels before birth, have lower (more masculine) 2D:4D on average[31][32][47] along with other possible physiological effects such as an enlarged clitoris and shallow vagina.[48] Males with CAH also express lower digit ratios than controls.[31][32] Amniocentesis samples in males with CAH show that prenatal levels of testosterone are in the high-normal range and levels of the weaker androgen androstenedione are several fold higher than in controls,[49][50][51] indicating that males with CAH are exposed to greater prenatal concentrations of total androgens.

A greater (more feminine) digit ratio occurs for men with Klinefelter's syndrome, who have reduced testosterone secretion throughout life compared to their fathers or to controls.[34]

Digit ratio in men may correlate with genetic variation in the androgen receptor gene.[52] Men with genes that produce androgen receptors that are less sensitive to testosterone (because they have more CAG repeats) have greater digit ratios, though there have also been reports of failure in replicating this finding.[53] Men with less sensitive androgen receptors may compensate for this by secreting more testosterone via reduced inhibitory feedback on gonadotropins.[54] Thus, it is not clear that 2D:4D would be expected to correlate with CAG repeats, even if it accurately reflects prenatal androgen.

XY individuals with androgen insensitivity syndrome due to a dysfunctional gene for the androgen receptor present as women and have greater digit ratios on average, as would be predicted if androgenic hormones affect digit ratios. This finding suggests that the sex difference in digit ratios may be unrelated to the Y chromosome per se.[55]

Geographic and ethnic variation

[edit]

Several studies have presented evidence that digit ratios are heritable.[56][57]

Manning and colleagues have reported that 2D:4D ratios vary greatly between different ethnic groups. In a study of Han, Berber, Uygur and Jamaican children, Manning et al. found that Han children had the highest mean values of 2D:4D (0.954±0.032). They were followed by the Berbers (0.950±0.033), then the Uygurs (0.946±0.037) and the Jamaican children had the lowest mean 2D:4D (0.935±0.035).[2][58] This variation is far larger than the differences between sexes; in Manning's words, "There's more difference between a Pole and a Finn, than a man and a woman."[59]

In 2007, Manning et al. reported further evidence of ethnic variation in mean 2D:4D, with higher ratios for Whites, non-Chinese Asians and Middle-Easterners, and lower ratios in Chinese and Black samples.[60] A 2008 study by Lu et al. found that the mean values of 2D:4D of the Hui and the Han in Ningxia were lower than those in European countries like Britain.[61]

Two studies explored the question of whether geographical differences in 2D:4D ratios were caused by gene pool differences or whether some environmental variable associated with latitude might be involved (e.g., exposure to sunlight or different day-length patterns). The conclusions were that geographical differences in 2D:4D ratio were caused by genetic pool differences, not by geographical latitude.[62][63]

Consanguineous parentage (inbreeding) has been found to lower the 2D:4D ratio in offspring,[64] which may account for some of the geographical and ethnic variation in 2D:4D ratios, as consanguinity rates depend on, among others, religion, culture, and geography.[65]

Cognitive and physical traits

[edit]

The 2D:4D ratio has been associated with a variety of traits including academic ability, personality as well as physical attributes such as health outcomes. Lower (masculinized) 2D:4D ratios have been associated with higher mathematical ability as well as personality traits such as aggressiveness (in males), assertiveness (in females) and reduced agreeableness.[1][66][67] Males with higher (feminized) ratios are reported to be more at risk of low sperm counts, heart disease, obesity, metabolic syndrome and depression than those with lower ratios.[68][69][70] Females with higher ratios are more at risk of eating disorders, anxiety and depression.[71]

Prenatal androgen exposure has been implicated as a determinant in left-handedness, though contradictory and inconclusive findings have been reported concerning an association between 2D:4D and left-hand dominance.[72][73][74] Kumar et al. have reported that hand preference is most strongly associated with 3D:4D digit ratios and that the effect is reversed between children and adults.[75]

Fink et al. (2004) found that men with low 2D:4D ratios (indicating high testosterone) and women with high 2D:4D ratios (indicating high estrogen) express greater levels of facial symmetry.[76] The prenatal sex hormone ratio (indicated by 2D:4D) was associated with similar alterations to both male and female face shapes in utero, though the effect was three times more pronounced in males than in females.[77]

In a non-clinical sample of women, digit ratio correlated with anogenital distance in the expected direction. In other words, women with a greater anogenital distance, indicating greater prenatal androgen exposure, had a smaller digit ratio.[78] Ronalds et al. (2002) showed that men who had an above average placental weight and a shorter neonatal crown-heel length had higher 2D:4D ratios in adult life.[79]

In 2020, a paper by John T Manning and Bernhard Fink reported that at the national level, high mean male 2D:4D was associated with high case fatality rates with COVID-19, and suggested that this may be used to "identify for whom it would be advisable to exercise social distancing."[80] Subsequent researchers failed to replicate their findings,[81][6] and criticised Manning and Fink for publishing papers that fail under scrutiny despite the urgent need for high quality science during the pandemic that informs medical decisions.[6]

Explanation of the digit ratio effect

[edit]

The mechanisms by which prenatal hormones influence digit ratio are not clearly understood. Traits such as otoacoustic emissions and arm-to-trunk length ratio are also influenced by prenatal hormones, and it is postulated that Hox genes responsible for both digit and penis development play an important role in affecting these multiple traits (pleiotropy).[82] Direct effects of sex hormones on bone growth might be responsible, either by regulation of Hox genes in digit development or independently of such genes.

Though it is unclear why the digit ratio on the right hand should be more sensitive to prenatal hormones than that on the left hand, as indicated by the greater sex difference on the right than the left,[83] these differences are not evident in dorsal measurements[84] nor bone digit ratios,[85][21][86] and it is possible that right-left differences in palmar 2D:4D may be attributed to differential placing of flexion creases.[11]

Criticism

[edit]

The credibility of research into the relationship between digit ratios and traits has been subject to criticism for various reasons.

  1. Weaknesses in the statistical significance of numerous findings.[87][88][89]
  2. Irreproducibility of findings in many instances, with some studies reporting contradictory results.[90][5][91][92][93]
  3. Lack of a clear cause-and-effect relationship between digit ratio and traits.[94][88][7]

Concerning the evidence for causal relationships, the reliability of digit ratio as a proxy variable has come under scrutiny.[5][94][87] Some studies that investigate the relationship between a trait and an unobservable variable fail to consider possible confounding variables.[95] This is especially problematic in cases where the relationship is likely to be weak, such as with prenatal testosterone.[96][92][87] Furthermore, critics have pointed out that controlling for confounding variables is an impossible task due to the expanding list of confounding variables, including ethnicity, sex,[58][97] and physical, medical or behavioral conditions.[98]

A 2021 paper by James Smoliga, titled "Giving science the finger—is the second-to-fourth digit ratio (2D:4D) a biomarker of good luck? A cross sectional study" followed similar research methodologies adopted by others in the field and came to conclude that low digit ratio is associated with good luck.[7] This result is attributed to chance and presented as an illustration of the reproducibility crisis. Smoliga would later go on to label the field as pseudoscience.[99] In a response, John T. Manning criticized the researchers for not controlling for ethnicity.[100]

Researcher Marta Kowal states: "One of the general limitations of studies on the digit ratios is the high number of degrees of freedom within the 2D:4D (the right, left, and right minus left 2D:4D). Running many analyses with different predictors increases the chances of finding allegedly significant results."[4]

Other animals

[edit]
  • Dennis McFadden and collaborators have demonstrated sexual dimorphism in hind limb digit ratio in a number of great apes, including gorillas and chimpanzees.[25]
  • Emma Nelson and Susanne Shultz are currently investigating how 2D:4D relates to primate mating strategies and the evolution of human sociality.[101]
  • Nancy Burley's research group has demonstrated sexual dimorphism in zebra finches, and found a correlation between digit ratio in females and the strength of their preference for sexually selected traits in males.[102]
  • Alžbeta Talarovičová and collaborators found in rats that elevated testosterone during the prenatal period can influence 4D length, the 2D:4D ratio, and open field motor activity.[103]
  • Peter L. Hurd, Theodore Garland Jr., and their students have examined hindlimb 2D:4D in lines of mice selectively bred for high voluntary wheel-running behavior (see experimental evolution). These high-runner mice exhibit increased 2D:4D in contradiction to the correlation between digit ratio and physical fitness in human beings, suggesting that 2D:4D may not be a clear proxy for prenatal androgen exposure in mice. The authors suggest that 2D:4D may more accurately reflect the effect of glucocorticoids or other factors that regulate any of various genes.[104]
  • In pheasants, the ratio of the 2nd to 4th digit of the foot has been shown to be influenced by manipulations of testosterone in the egg.[105]
  • Studies in mice indicate that prenatal androgen influences 2D:4D primarily by promoting growth of the fourth digit.[30]

See also

[edit]

References

[edit]
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  2. ^ a b Manning JT, Barley L, Walton J, Lewis-Jones DI, Trivers RL, Singh D, et al. (May 2000). "The 2nd:4th digit ratio, sexual dimorphism, population differences, and reproductive success. evidence for sexually antagonistic genes?". Evolution and Human Behavior. 21 (3): 163–183. Bibcode:2000EHumB..21..163M. doi:10.1016/S1090-5138(00)00029-5. PMID 10828555.
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