Why do film noir movies use high-contrast lighting and deep shadows?

In film noir, high-contrast lighting and deep shadows serve as the genre's most notable visual features, functioning primarily to underscore the isolation of the typical noir hero and to heighten the sombre, cynical tone of the narrative.1

These stylistic choices trace their origins to German Expressionist cinema, where directors used dark atmospheres and skewed visual elements to externalize internal states, such as madness or psychological distress. When adopted by Hollywood, this low-keyed lighting was used to reflect the broader cultural disenchantment of the post-World War II era.12

The darkness of these films embodied a collective sense of uncertainty, pessimism, and disillusionment following the Great Depression and the war. By creating a claustrophobic and corrupt world, these lighting techniques helped directors convey the feeling that the characters were trapped in a heartless, modernized society. Ultimately, these shadows were essential for establishing the genre's signature mood of uncertainty and hopelessness.213