With mass killing shootings in the news, I’d like to point out that not all bad things are destined to increase forever. For instance, according to the Radford University Database of known serial killers, the number of serial killers soared during what Robert Heinlein predicted c. 1940 would be known as the Crazy Years (1960s-1970s) before declining more recently.
One thing to keep in mind is that some of the sharp decline is due to the rule that the serial killer must be caught before being counted. Sometimes it can take decades to catch a cautious killer, as seen in the recent arrest of some old-time serial killers betrayed by relatives publishing their DNA online, such as the Golden State Killer (active 1974-1986). So, the decline is partly an artifact of that.
On the other hand, it’s almost certainly true that serial killings are in decline. The phenomenon of the serial killer doesn’t take up as much space in our brains as in, say, the 1990s. My vague impression is that 21st century serial killer movies (e.g., Zodiac) are often period pieces.
It appears that the idea of serial killing was something of a social contagion that spread first among whites, then among nonwhites. I wouldn’t be surprised if Hitchcock’s hugely influential 1960 movie Psycho, often thought as the founder of the “slasher pic” genre, played a role in this real life phenomenon, although how to measure that is beyond me.
It’s also hard to say what caused the decline over the last generation. It could be that serial killing became less appealing to the handful of sickos attracted to doing it.
Or it could be fear of being caught increased. According to Bill James, cops were long particularly bad at catching serial killers because they’d been trained not to fall for the idea that somebody was murdered by a random stranger: instead, it had to be somebody who knew the victim, an ex-boyfriend or the like. So if they had five dead women on their hands, they tended to look for five separate killers. This had been a fairly productive prejudice, since it kept them from going down the wrong path most of the time. But the huge publicity attendant to Ted Bundy c. 1980 forced cops to get serious about the serial killer phenomenon. (This means that some of the pre-1980 serial killers might have been missed for this graph.)
Nowadays, we have lots of TV shows about supergenius scientific detectives using their high-tech labs to catch serial killers, which might serve to deter would-be serial killers, who, unlike mass killing shooters, want to get away with their crimes.
In contrast, a lot of current true-crime murder shows, both documentary and acted-out documentary (Renée Zellweger’s The Thing About Pam), devote themselves these days to murders over rather small life insurance policies.
In summary, not everything bad increases forever.