Rick Nevin‘s research provides a lot of evidence that the amount of violent crime — murders, aggravated assault, etc. — are the result of lead pollution. Lead was added to gasoline until the 1970’s. When the gasoline was burned in car engines, the lead was released into the atmosphere where it could get into people’s systems just by breathing.
Quite a number of studies taken together have shown that high blood lead levels result in lower IQ’s, which, in turn, seems to increase aggressive behavior.
Long-term trends in paint and gasoline lead exposure are also strongly associated with subsequent trends in murder rates going back to 1900. The findings on violent crime and unwed pregnancy are consistent with published data describing the relationship between IQ and social behavior. The findings with respect to violent crime are also consistent with studies indicating that children with higher bone lead tend to display more aggressive and delinquent behavior.
— Nevin (2000): How Lead Exposure Relates to Temporal Changes in IQ, Violent Crime, and Unwed Pregnancy (pdf pre-print) in Environmental Research.
Kevin Drum summarizes the research and goes into the details to disprove the other theories for peaks in crime rates in the last century.