The author begins by acknowledging the difficulties in defining film noir. Contrasting it with other established genres like horror or western, Schrader reckons that the differentiating quality of film noir is its subtle yet dark tone and mood. More than qualities inherent to the film, its periodic setting and its production in the forties and early fifties are better markers of the genre. There were four key socio-political conditions during this period which were instrumental in the birth of film noir. The first was war and post-war disillusionment. The second was the gaining popularity of post-war realism. Thirdly the German influence on Hollywood was getting traction. Finally, the growing prominence of “hard-boiled” school of writers within mainstream cinema. What also distinguishes film noir is its stylistic elements and themes. In terms of style, the dominant night scenes, elements of German expressionism, lighting emphasis on both characters and props, as well as the presence of compositional tension, are all unique to film noir. One can also add the complex chronological ordering of narration to the list. In sum, in my opinion, author Paul Schrader does a commendable job of getting to the heart of the cinematic genre. His systematic way of identifying and articulating characteristics unique to the genre is particularly impressive.
Paul Schrader, Notes on Film Noir (1972), Film Noir Reader, p.53-63.