A thing which comes up in murder mysteries about clever murders (that is, murders which were cleverly planned and executed, as opposed to those merely covered up well) is the murderer using the same technique again and being sloppier the second time (or the third or fourth). There is an interesting psychological insight in this.

The first time a person tries something, they’re new to it and everything is scary. When it succeeds, they then evaluate how it went. Most people do this primarily to figure out how to improve, but one naturally also takes stock of where one spent unnecessary work in order to streamline the process. The same thing applies to murderers. They pay attention to what people noticed but couldn’t figure out and take extra care to not leave those clues. But they also can’t help but be aware of what no one noticed. Human nature being what it is, it will be very hard for them to put the same level of effort into covering up things that weren’t a problem last time, and they will likely leave these same clues again, possibly even stronger. Maybe no one noticed the cigarette butt that they forgot to pick up; they will be all the more likely to not remember to pick up the cigarette butt next time.

But what the murderer can’t know is why no one noticed. Perhaps they didn’t notice by accident. Perhaps they did notice but thought it didn’t mean anything because it could too easily be a coincidence. Perhaps they noticed and there was some circumstance the murderer didn’t know about that explained the clue away.

To continue with the example of the cigarette butt, in the first case, maybe the cigarette butt was dropped in a place where it blended in or it was disguised by a leaf falling over it or by rain distorting the paper and making it look older than it was. In the second murder, the cigarette butt might be dropped in a place where it stood out more and got noticed. In the second case, suppose it was a relatively common brand of cigarette, though not super common. Showing up in one place barely registers because it might have been anyone. But showing up in both places seems far less likely and attracts attention. In the third case, suppose that, unknown to the murderer, the first victim smoked the same brand of cigarettes as the murderer. He would have thought that the butt was not noticed, when in fact it was noticed and it was only a coincidence that it was not thought important.

The first and third of these may or may not apply to any given repeat of a murder technique, but the second of them necessarily does. Or at least it necessarily does if someone believes that they two murders are connected. When the detective believes that the two murders are connected, he will begin to look for similarities between them, which makes a different set of facts stand out than when investigating just one.

A very good example of this which comes to mind is in Three Act Tragedy. (spoilers follow) The murderer actually tried out his method of murder once in a condition in which he was very protected, which did show a kind of good sense in that, if there were flaws in the method, it would have been revealed to him when it was virtually impossible to bring home the crime to him. But what he didn’t count on was that the act of testing out the method of murder produced two instances of the method being used which could be compared; this produced clues which would not have existed otherwise. And, unfortunately for the murderer, one of the main connections between the two events was him. It’s true that he took pains to conspicuously not be at the scene of the second murder, but the second victim was so connected to him that the connection could not be avoided.

There is also, of course, the problem that murderers never count on the fact that the more times you try something, the more likely you are to eventually be unlucky. In Three Act Tragedy it is a pure accident that a doctor who recognizes the symptoms of nicotine poisoning happens to be present when the victim dies.

An interesting corollary to this is that a good way to use a “perfect” murder technique in a detective story that still allows the detective to catch the murderer is to have the murderer use it more than once. That lets you enjoy the cleverness of the technique without ruining it by making it done poorly or not actually that good. It allows the murderer to be brilliant—in devising the murder—and his flaw to be his bad judgement and/or laziness. Either goes well with the flaws intrinsic to a murderer.