This article is a couple of years old, but it's the best explanation I've seen yet of the evidence that reducing lead exposure reduces crime.
Although crime did fall dramatically in New York during Giuliani's tenure, a broad range of scientific research has emerged in recent years to show that the mayor deserves only a fraction of the credit that he claims. The most compelling information has come from an economist in Fairfax who has argued in a series of little-noticed papers that the "New York miracle" was caused by local and federal efforts decades earlier to reduce lead poisoning.
The theory offered by the 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.
Bonus question: what does this say about morality and free will?