One man's studies suggests that lead poisoning rates are closely linked to violent crime rates twenty years later. More analysis of the study by Rick Nevin is found here, and an abstract of the May 2000 article is found here.
He attributed the big 1990s drop in violent crime to the end of leaded gasoline, and previous crime surges to increased use of lead. He argues that lead is the lead exposure is a leading cause of violent crime.
The link isn't obvious, but a route of causation is plausible, and the data are rather strong. The alternative, of course, is that lead exposure could be a proxy for other variables that are actually causes, like poverty.
The news is basically good, because lead exposure reductions have been one of the great success stories of recent environmental history. The primary exposure rate at this point is through old lead pipes in older cities (Washington DC among them).