In 1926, Agatha Christie published a short story that introduced the character of Miss Marple, a spinster who solved crime by the ability to talk to anyone, by being the sort of person no-one pays attention to, by her insights into normal human behaviour.
There are big differences between the characters, but they have a lot in common as well. I suspect Sayers must have been writing her novel at the same time that Christie’s story appeared. I do not suspect that one influenced the other. I think it is pure coincidence. But it is the coincidence that is interesting.
Sayers would often write insightful novels about social conditions disguised as crime novels. The Unpleasantness at the Bellona Club (which came out just a year after Unnatural Death) is a telling account of the long-term social and psychological effects of the First World War; Murder Must Advertise is not just one of the best novels about advertising that I know, it is also very sharp on the profession’s ethics. And Unnatural Death fits into this model; without making any fuss about it, it is a novel about the position of women in post-First World War England. It is a novel crowded with women, and with very few men; there are at least two lesbian relationships within the novel though nothing much is made of either of them. We see a social world that is almost entirely made up of women, there are spinsters everywhere, there are independent women (including both the victim and the murderer), there are women faced with new freedoms and not at all sure what to do with them.
The First World War destroyed a generation; that has been a cliché of British history, probably since the end of the war itself. It is only recently, it seems, that historians have started to look at what that meant for the women of Britain. But that meaning, that effect, was there in the novels of the time, and it is overt in this novel by Sayers. I suspect that Unnatural Death and The Unpleasantness at the Bellona Club can be read as a dyptich on the consequences of the war, but it is the women who come first.
Christie was always more mechanistic, less socially astute, than Sayers. Her plots are well-oiled machines, but the world around those plots is a never-never land, unreal and uninteresting. But she was still living in the world, she cannot have been totally unaware of how things were, and I suspect that Miss Marple grew out of the fact that the 1920s was a world of women not of men. Miss Marple and Miss Climpson may both have been gifts to crime fiction, but they were both victims of the First World War.