The actual set of contexts that license particular polarity items is not as easily defined as a simple distinction between affirmative and negative sentences.
To complicate matters, polarity items appear to be highly idiosyncratic, each with its own set of licensing contexts.
Early discussion of polarity items can be found in the work of Otto Jespersen and Edward Klima.
In the late 1970s, William Ladusaw (building on work by Gilles Fauconnier) discovered that most English NPIs are licensed in downward entailing environments.
Nor is a downward entailing environment a sufficient condition for all negative polarity items, as first pointed out by Zwarts (1981) for Dutch "ook maar."