My oldest son and I recently watched The A.B.C. Murders, and at the end there was a part, as Poirot was detailing the evidence against the murderer, where he added that a fingerprint of the man Poirot was accusing was found on the typewriter that the murderer used. Later, Hasting commented that the fingerprint produced a strong effect (the suspect tried to commit suicide).

That fingerprint clinched things, Poirot,” I said thoughtfully, “He went all to pieces when you mentioned that.”
“Yes, they are useful—fingerprints.”
he added thoughtfully:
“I put that in to please you, my friend.”
“But, Poirot,” I cried, “wasn’t it true?”
“Not in the least, mon ami,” said Hercule Poirot.

One of the curious things about detective fiction is that it comes on the scene almost contemporaneously with the advent of forensics, the use of technology to catch crimes, and police forces organized in the modern manner. Francis Galton only published his statistical analysis that established fingerprints as a viable means of unique identification by the police in 1892. The first arrest and conviction of someone on the basis of fingerprint evidence was ten years later, in 1902. The golden age of detective fiction, if we include Sherlock Holmes in it (which we should), begins before the use of fingerprints as evidence in crimes.

As I mentioned in Fingerprints in Detective Stories, it’s not difficult to see why fingerprints are almost never used as real evidence in detective stories. We want detective stories to be interesting and the detective to be brilliant. “There was a fingerprint on the dagger in the victim’s back, we checked it against everyone’s fingerprints, it turns out to belong to his brother, therefore the brother is the murderer, the end” isn’t much of a story, and doesn’t require a brilliant detective.

Which actually brings me to the relationship between forensic science and fingerprints, because it is interesting to consider that while fingerprints were rarely used in detective stories, plenty of golden age detective stories were primarily about forensic science. Sherlock Holmes was often conducting scientific experiments to prove a case, though to my recollection rarely as the main story. This may have reached its apotheosis in Dr. Thorndyke. I’ve read that when the short stories were published they would include photographs of what the good doctor would have seen through his microscope as described in the story, and other such things. Thorndyke also made extensive use of enlarging photography and other forensic technologies. The stories have faded, considerably, in the public’s memory—to some degree the fate of everything whose main attraction was being on the cutting edge of science or technology. They were, so I read, immensely popular at the time. Their role is probably taken, these days, by police procedural television shows, whose stock and trade is often the cutting edge of forensic science.

I can’t help but wonder if it was G.K. Chesterton’s Father Brown that helped to move detective fiction from a focus on forensics to include psychology. Chesterton first wrote Father Brown in 1910, which was still early on in the golden age. To be sure, more than half of Sherlock Holmes had been written by then and Holmes was no slave to forensics nor was he ignorant of human psychology. Still, he was an expert. He could identify over one hundred brands of cigar by their ash and could tell where a patch of mud on the trousers was picked up in london by its composition, just from looking at it.

Father Brown was not an expert—at physical details. We was an expert in the human being, which proved far more interesting.

This move to psychological mysteries brought with it what has, I think, made the murder mystery so enduring: the puzzle. Once forensics were established as a norm, murderers began to use their cunning to fake the forensic evidence and lead the forensic detectives astray. The psychological detective was necessary to combat this newer breed of criminal. It was at once more interesting and also more accessible. It is not really worth anyone’s time to minutely study cigar ash, but anyone can (if sufficiently clever) figure out the meaning of a particular kind of cigar ash being found in a particular place.

Poirot very much represents this transition. He said many times that he does not get on his hands and knees to find the clues, as anyone can do that. His job is to understand what the clues mean. The A.B.C. Murders was published in 1935, when the fascination with forensic detection was still fresh. It’s curious to see traces of this in the Poirot stories.