Cosy and Noir are usually seen as the two extremes bookending the large spectrum of crime fiction. That claim is not without merit (they are indeed extremes) but also rests on the mistaken premise that Cosy and Noir are separate, distinct genres – which they are not.
This may seem a rather counter-intuitive take as both Cosy and Noir come with a whole set of characteristics, conventions and expectations that make them immediately identifiable. If your book’s main protagonist is a cat, then obviously it is a cosy; if on the other hand it is a crook at the end of his rope then it’s more than probably noir. I don’t dispute that, for my point is not that Cosy and Noir don’t exist but that they are more accurately called moods than genres.
There is nothing in a « feline » mystery that makes it fundamentally different from a traditional, human-led one; both share the same basic structure and assumptions and often the same readership. Like it or not, The Hound of the Baskervilles and The Chihuahua of the Baskervilles are both detective stories; the only difference is that one takes itself and the genre seriously whereas the other does not. Conversely, what separates Double Indemnity from contemporary crime novels like Malice Aforethought is a matter of setting and tone, not a difference of nature.
The Cosy/Noir divide can be found all across the spectrum of crime fiction, not just at its ends. The Murder at the Vicarage is a Golden Age mystery, but so is the much bleaker Lonely Madgalen, and the hardboiled school was home to Philip Marlowe but also welcomed Richard S. Prather.
Perhaps the most striking example is that of Cornell Woolrich. His extremely dark and misanthropic worldview has made him one of the most famous Noir writers but his chosen narrative idiom was that of psychological suspense, a genre that also gave us the uber-optimistic, love-thy-neighbour Charlotte Armstrong. Once again the difference is largely one of tone, for they otherwise worked from the same material – fear.
Crime fiction is extremely diverse but in the end can be divided into three main categories depending on whether it adopts the viewpoint of the Detective, the Criminal and the Victim. These I think are enough to cover all bases. This doesn’t mean Cosy and Noir don’t have their uses but that said uses are descriptive, rather than categorizing, ones. They say a lot about the mood of a book but not necessarily much about the book itself.