Undoubtedly lead exposure has had a profound effect on society, and on crime levels. Lead exposure leads to reduced IQ and increased aggression. But the crime l

Undoubtedly lead exposure has had a profound effect on society, and on crime levels. Lead exposure leads to reduced IQ and increased aggression. But the crime landscape during the relevant period (the late 20th century) has too many confounding factors to extract data from it. For one, there's the "crack epidemic" crime wave of the late '80s and early '90s. During this period the rate of homicides committed by teens tripled and then went back to previous levels. Additionally, violent crime is correlated strongly with population density. And during the late 20th century in the US cities first began to empty out (due to "white flight") and then began to be repopulated as "yuppies" working office jobs and living in cities became more a more desirable "american dream" than the suburban family home and then later as urban centers became the core of the new tech economy. At the same time, police enforcement changed dramatically through the '80s, '90s, and later. The "war on (some) drugs (used by some people)", mass incarceration, etc. These are all tremendous confounding factors while the underlying "signal" from lead exposure is expected to be a gradual change over a fairly long period.