Related fun fact: It’s considered a real possibility that the rates of homicide, and, in particular, the pervasiveness of serial killers in the US throughout the mid to late 20th century may have a direct connection to the increase in ubiquity of automobile use in post-WWII America along with the use of leaded gasoline. The idea is that as more leaded gasoline fumes suffused the air, low level lead poisoning was raising the general aggression levels and psychosis in the American population. In those who were already inclined toward violence or psychotic behavior, this may have pushed them over the edge.

Unleaded gasoline became available in the 70’s and leaded gasoline was phased out over time until it was completely gone by 1996. That would help explain the very large decades long rise and fall of serial killer murders. Between the 30’s and the 60’s serial murders were, by comparison to its peak in the 70’s-80’s, incredibly uncommon. Single digits percentages by comparison. There was a steady increase in serial killer activity starting around the late 60’s when Boomers were entering their late teens and twenties and multicar households became common. There was leveling off of serial murder rates through the mid 80’s to the mid 90’s as leaded gas was being phased out. And then the steady decline in serial murder since leaded gasoline was completely gone in the mid 90’s. The changes in overall homicide rates follow a similar pattern, though the changes are a bit less pronounced.