This sub-genre was born in Los Angeles, California, USA. In the early to late 1930s, when life was hard, and the threat of war and financial depression/austerity was hitting everyone, somewhat like today’s USA and UK.
People were moving there to write, and some lost out. Whereas, Dashiell Hammett and Raymond Chandler told stories that hit the heart of real life with bitter, angry and drunken detectives. Where their investigators used their intelligence and intellect, looking for clues in everyday life and observed normal human nature.
There was no side-kick as in many detective fictions, to talk matters over for the audience’s understanding. Such as Sherlock Holmes and Dr Watson. Inspector Morse and Sergeant Lewis. Hammett and Chandler used the first person so they could integrate Philip Marlowe, Mike Hammer and Sam Spade’s thoughts into the story.
There was usually a murder, but not much violence occurred after the discovery of a dead body.