Such a correlation does not prove that lead had any effect on crime levels. But in an article published this month in the B.E. Journal of Economic Analysis and Policy, Reyes uses small variations in the lead content of gasoline from state to state to strengthen her argument. If other possible sources of crime like beer consumption and unemployment had remained constant, she estimates, the switch to unleaded gas alone would have caused the rate of violent crime to fall by more than half over the 1990s.

If lead poisoning is a factor in the development of criminal behavior, then countries that didn’t switch to unleaded fuel until the 1980s, like Britain and Australia, should soon see a dip in crime as the last lead-damaged children outgrow their most violent years. According to a comparison of nine countries published this year by Rick Nevin in the journal Environmental Research, crime rates around the world are just starting to respond to the removal of lead from gasoline and paint. “It really does sound like a bad science-fiction plot,” says Nevin, a senior adviser to the National Center for Healthy Housing. “The idea that a society could have systematically poisoned its youngest children with the same neurotoxins in two different ways over the same century is almost impossible to believe.”

The magnitude of these claims has been met with a fair amount of skepticism. Jeffrey Miron, a Harvard economist, wonders how lead could have had such a strong effect on violent crime while, according to Reyes, it showed almost no effect on property crimes like theft. He also doubts that the hypothesis could explain the plunge in the U.S. murder rate from the 1930s through the 1950s. “I certainly think it’s a reasonable exercise,” Miron says. “We just have to be appropriately suspicious of how much you can actually show.”

The theory will be put to the test as children grow up in Indonesia, Venezuela and sub-Saharan Africa, where leaded gasoline has just recently been phased out. Meanwhile, the list of countries that still use lead in gas — Afghanistan, Serbia and Iraq, as well as much of North Africa and Central Asia — does not rule out a connection with violence.

No matter how suggestive the economists’ data, it takes a doctor to show that some of the people most damaged by lead are out there breaking the law. Herbert Needleman, the University of Pittsburgh psychiatrist and pediatrician whose work helped persuade the government to ban lead in the 1970s, recently studied a sample of juvenile delinquents in Pittsburgh; the group had significantly more lead in their bones than their peers. And lead may not be the only source of damage. The National Children’s Study will soon begin to track more than 100,000 children to determine the effects of exposure to common pesticides, among other chemicals.