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This sort of relationship has been found again and again, not just in the US but in countries around the world. Lead exposure at a young age is known to reduce IQ and increase impulsivity; expose children to lead, and you increase their rate of criminality in young adulthood. This has been found empirically, quote: "Groups of children have been followed from the womb to adulthood, and higher childhood blood lead levels are consistently associated with higher adult arrest rates for violent crimes." Quote: "A second study found that high exposure to lead during childhood was linked to a permanent loss of gray matter in the prefrontal cortex--part of the brain associated with aggression control as well as what psychologists call 'executive functions': emotional regulation, impulse control, attention, verbal reasoning, and mental flexibility."
Lead exposure and IQ drop:
The linked article mentions numerous alternate explanations for the drop in crime: drug use, incarceration rates, 'broken windows', legal abortion, demographic shifts, economic factors...some of these are more plausible than others, but none has the degree of evidence as does lead exposure. I highly recommend reading the whole thing! It's very compelling, and concludes with an argument for further action on lead abatement, to additionally reduce lead exposure levels.
Oops, I mean roughly when criminality peaks.