Studies between cities, states and nations show that the rise and fall in crime follows, with a roughly 20-year lag, the rise and fall in the exposure of infants to trace quantities of lead.

...The curve is much the same in all the countries these papers have studied. Lead was withdrawn first from paint and then from petrol at different times in different places (beginning in the 1970s in the US in the case of petrol and the 1990s in many parts of Europe), yet, despite these different times and different circumstances, the pattern is the same: violent crime peaks around 20 years after lead pollution peaks. The crime rates in big and small cities in the US, once wildly different, have now converged, also some 20 years after the phase-out.

Nothing else seems to explain these trends. The researchers have taken great pains to correct for the obvious complicating variables: social, economic and legal factors. One paper found, after 15 variables had been taken into account, a four-fold increase in homicides in US counties with the highest lead pollution. Another discovered that lead levels appeared to explain 90% of the difference in rates of aggravated assault between US cities.

A study in Cincinnati finds that young people prosecuted for delinquency are four times more likely than the general population to have high levels of lead in their bones. A meta-analysis (a study of studies) of 19 papers found no evidence that other factors could explain the correlation between exposure to lead and conduct problems among young people.

Is it really so surprising that a highly potent nerve toxin causes behavioral change? The devastating and permanent impacts of even very low levels of lead on IQ have been known for many decades. Behavioral effects were first documented in 1943: infants who had tragically chewed the leaded paint off the railings of their cots were found, years after they had recovered from acute poisoning, to be highly disposed to aggression and violence.

Lead poisoning in infancy, even at very low levels, impairs the development of those parts of the brain (the anterior cingulate cortex and prefrontal cortex) which regulate behavior and mood. The effect is stronger in boys than in girls. Lead poisoning is associated with attention deficit disorder, impulsiveness, aggression and, according to one paper, psychopathy. Lead is so toxic that it is unsafe at any level.

Because they were more likely to live in inner cities, in unrenovated housing whose lead paint was peeling and beside busy roads, African Americans have been subjected to higher average levels of lead poisoning than white Americans. One study, published in 1986, found that 18% of white children but 52% of black children in the US had over 20 milligrams per deciliter of lead in their blood; another that, between 1976 and 1980, black infants were eight times more likely to be carrying the horrendous load of 40mg/dl. This, two papers propose, could explain much of the difference in crime rates between black and white Americans, and the supposed difference in IQ trumpeted by the book The Bell Curve.