Polarity item

In linguistics polarity item is an expression which is sensitive to the presence, in the same sentence, of certain other expressions that are "somehow negative". Polarity items divide into those who must co-occur with a "somehow negative" expression ("negative polarity item", NPI) and those who cannot ("positive polarity item", PPI). An example of an NPI is the English word any. It is ungrammatical if it occurs in a sentence without a "negative" expression (A star "*" in front of a sentence means the sentence is odd or ill-formed):

John doesn't have any potatoes.

*John has any potatoes.

One says of an NPI like any that it is licensed by a negative expression. An example of a PPI is the English word somewhat. If it occurs in a sentence with another negative expression, the sentence is ill-formed:

John liked it somewhat.

*John didn't like it somewhat.

One says of a PPI like somewhat that it is anti-licensed by a negative expression. Early discusion of polarity items can be found in the work of Otto Jespersen and Edward Klima.

Much of the research on polarity items has centerd around the question what it takes for an expression to be "somehow negative". NPIs like any. In the late seventies Bill Ladusaw discovered that most NPIs are licesed in downward entailing environments. While it is widely accepted that this is a sufficient criterion for an NPI to be licensed, it is a matter of current debate whether it is also a necessary criterion.