Here is what the experts have to say. Take a look at the writings of Gary Lovisi and Richard Golsan and their analysis of the genre
http://muse.jhu.edu/journals/scr/summary/v027/27.1-2.golsan.htmlhttp://www.gryphonbooks.com/Articles/hard_boiled_way.html
http://muse.jhu.edu/journals/scr/summary/v027/27.1-2.golsan.htmlhttp://www.gryphonbooks.com/Articles/hard_boiled_way.html