As Paul Schrader notes with reference to a statement by Raymond Durgnat, film noir is not a genre and is not defined in terms of conventions of setting and conflict. Instead, it is defined by the subtler qualities of tone and mood and is also defined by its time period:
In general, film noir refers to those Hollywood films of the forties and early fifties that portrayed the world of dark, slick city streets, crime and corruption (Schrader 170).
Film noir was itself a system of visual and thematic conventions which were not associated with any specific genre or story formula, but rather with a distinctive cinematic style and a particular historical period (Schatz 112).
Schatz's insistence on noting the relationship to a historical period is important because it indicates that film noir was a social, psychological, and aesthetic response to a certain sense of societal angst that developed first in the uncertainties of World War II, a period of world tensions manifested in the psychological ambiguities of film noir, and then continued in the new uncertainties of the Cold War period, especially in the years immediately after World War II when American society was straining to recover from the war while also trying to adapt to the new line-up of international friends and enemies. This also explains why the style became so pervasive, since it was speaking to the national psyche that existed at the time, and that psyche did not kick in only for one genr