Film Noir

This is a film type I've been drawn to lately. I was wondering how people define it. This one source said film buffs or experts have differing definitions of it. But it says that dismal or negative themes came from the depression and also, since many deal with bad females, they draw from men around WW2 who returned to their wives to find they had changed, I guess not for the better. Their women were working more. It says some stories come from books written in the 1930s. Hitchcocks 'Rebecca' falls directly into that category, written by English female writer Daphne du maurier. They are usually in black and white,, for the visual effect, existing after color had come into film, although 'Body Heat', which is inspired by 1940s 'double indemnity' is truly a film noir in color. 'Suspicion,' by Hitchcock in black and white is about a male who is sinister yet would definitely seem to fit the genre. 

'Hush hush sweet Charlotte' is a classic film noir where a female cousin is trying to make her cousin crazy in order to get her money. 'Whatever happened to baby Jane' is about sisters. One sister has caused her sister a car accident and takes care of her invalid sister but is trying to kill her. This stems from envy bc the sister she's trying to kill became a bigger movie star than her though she had been the bigger star.