Could the cause be the clean-up of lead?
From a 2007 article from the Washington Post:
I am always suspicious of simple, monocausal theories of social change, but this one actually has some strong evidence behind it. By one estimate, a dollar spent on lead abatement back in the day saved more than a dollar in crime cost each year thereafter, which is ten times more impact than a dollar spent on prisons or police. Yet for some reason lead abatement has been portrayed as a soft, do-gooding liberal sort of thing, compared to being "tough on crime." And even if the lead clean-up is not responsible for 90% of the decline in crime, it has certainly prevented thousands of cases of mental retardation and other problems. Leaded paint and gasoline were environmental catastrophes.
The theory offered by economist Rick Nevin is that lead poisoning accounts for much of the variation in violent crime in the United States. It offers a unifying new neurochemical theory for fluctuations in the crime rate, and it is based on studies linking children's exposure to lead with violent behavior later in their lives.
What makes Nevin's work persuasive is that he has shown an identical, decades-long association between lead poisoning and crime rates in nine countries.
"It is stunning how strong the association is," Nevin said in an interview. "Sixty-five to ninety percent or more of the substantial variation in violent crime in all these countries was explained by lead."
Through much of the 20th century, lead in U.S. paint and gasoline fumes poisoned toddlers as they put contaminated hands in their mouths. The consequences on crime, Nevin found, occurred when poisoning victims became adolescents. Nevin does not say that lead is the only factor behind crime, but he says it is the biggest factor.