It could just be better policing. If you catch someone after they kill once or twice, they don’t get the chance to become a serial killer. But I haven’t looked up any stats on this.
If it was “better policing”, you expect the clearance rate for actual homicides to have gone up as well as the identified serial killers to have gone down. Instead, the clearance rate has gone down, as well. “Worse policing” of murder would better explain both reduced clearance of homicides and reduced numbers of identified serial killers.
Worse policing also leads to more incorrect clearings.
Who knows how many murders were incorrectly pinned on husbands or wives, or known petty criminals, or tramps? The same technology we use to find murderers can also exonerate people.
We also tossed out a lot of bogus methods based on pseudo-science and learned a lot about the reliability of witnesses; and all the modern technology exposed many expert witnesses as charlatans.
