As I mentioned, I’ve gotten a copy of Dorothy L. Sayers essay in the book Titles to Fame, in which she discussed the creation of her novel, Gaudy Night. I’ve read it over twice, and will be writing a more in-depth analysis of it, but at the moment I wanted to give some preliminary thoughts.
One of the things which leaps out at me is that she described the nature of detective stories in the early 1920s as being very focused on plot, to the exclusion of character. They were not supposed to be “serious”. Especially interesting to me is that she gave, as the exception that proved the rule, G.K. Chesterton’s Father Brown mysteries. He introduced philosophy into the detective story, but he was also an acknowledge master of paradoxes, and popular detective stories which were philosophical were simply one more paradox in his rather large bag of them.
She goes on to describe a trend, from the twenties into the thirties, of detective stories becoming more fleshed-out stories and less pure puzzles. This trend I find interesting, because, depending on whether you count the detective story as starting out with Sherlock Holmes or C. Auguste Dupin, it didn’t really start as a pure puzzle. In fact, even if you count the detective story as starting out with C. Auguste Dupin, you can still observe the trend of moving more toward pure puzzles, with the final Dupin story being entirely about reasoning from newspaper articles.
Be that as it may, it does raise an interesting question: why would people prefer detective stories that were pure puzzles, without real characters?
Before attempting to answer that, I think it worth noting that I’m not sure that Ms. Sayers was entirely correct. My evidence for this is hardly conclusive, but for example I can find no major support for it in the book Masters of Mystery, published in 1930. That was a few years too late to be in the full sway of what Ms. Sayers is describing, so it does not suffice. On the flip side of the 1920s, the 1907 The Red Thumb Mark and 1911 The Eye of Osiris Dr. Thorndyke novels were both almost as much love stories as they were detective stories. They are even further in time from the early 20s than Masters of Mystery was, though. The only thing with which I am familiar and which is right about that time is The Mysterious Affair at Styles, which was published in 1920 (in the United States; early 1921 in England). I am afraid I must confess that I haven’t actually read the book—I’ve only read a handful of the Poirot stories—I’ve only seen the David Suchet TV production of it. While it certainly is not a novel of manners, nor is it Gaudy Night, neither is it merely a crossword puzzle in literary form. That said, a handful of further exceptions will not disprove a general rule, and most of the detective fiction of the time period has been lost to us in the mists of time.
To return to the question at hand, I think that there is an excellent reason for detective stories to have moved, for a time, in the direction of pure puzzles: they were new and people had not yet worked out how to do the puzzles. This was true both of readers as well as writers; both were figuring out what the puzzle inside of a detective story was.
When something is new, there is, of course, the pleasure of novelty, but there is also the difficulty of novelty. The structures which make up the new thing are unfamiliar, which makes initial learning easy, but the unfamiliarity of the structures of the new thing also makes it hard to do anything else other than learn them. Accordingly, it makes sense to prefer the things in a purer form.
To give an example of what I mean, early on a person may not be suspected by the reader merely because his presence seems obvious, though it might not have been necessary. Once this is learnt, however, it becomes possible to trick the reader by casting suspicion on a character by the trick of obviously diverting suspicion from him. Once this trick is learnt, it becomes unclear what sort of trick is being played, and so the reader knows to suspend his judgement merely because a character appears innocent.
There are many such examples that can be given; writers and readers have gone through bluffs and double bluffs and triple bluffs, until finally the rules of the game have been pretty well learnt by both and it is interesting rather than taxing to add in other elements.
To put the thing in another way, it took a while for Fr. Knox’s Decalogue to come about. As I’ve argued elsewhere, the issue with fair play is not really about a guessing game between reader and writer, but rather that it keeps the writer honest and makes the story a much better detective story. Once the rules of detective fiction were worked out, the detective story became good enough to make alloys of it with other sorts of stories.
I do not know that this is what happened, of course, and still less do I know, if this did happen, that this is why it happened. If it did happen, though, this does seem to me the most likely reason why.