Has the Clean Air Act done more to fight crime than any other policy in American history? That is the claim of a new environmental theory of criminal behavior.

In the early 1990s, a surge in the number of teenagers threatened a crime wave of unprecedented proportions. But to the surprise of some experts, crime fell steadily instead. Many explanations have been offered in hindsight, including economic growth, the expansion of police forces, the rise of prison populations and the end of the crack epidemic. But no one knows exactly why crime declined so steeply.

The answer, according to Jessica Wolpaw Reyes, an economist at Amherst College, lies in the cleanup of a toxic chemical that affected nearly everyone in the United States for most of the last century. After moving out of an old townhouse in Boston when her first child was born in 2000, Reyes started looking into the effects of lead poisoning. She learned that even low levels of lead can cause brain damage that makes children less intelligent and, in some cases, more impulsive and aggressive. She also discovered that the main source of lead in the air and water had not been paint but rather leaded gasoline -- until it was phased out in the 1970s and '80s by the Clean Air Act, which took blood levels of lead for all Americans down to a fraction of what they had been."Putting the two together," she says,"it seemed that this big change in people's exposure to lead might have led to some big changes in behavior."