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Research Links Lead Exposure, Criminal Activity
"In Britain and most of Europe, they did not have meaningful constraints [on leaded gasoline] until the mid-1980s and even early 1990s," he said. "This is the reason you are seeing the crime rate soar in Mexico and Latin America, but [it] has fallen in the United States."
Lead levels plummeted in New York in the early 1970s, driven by federal policies to eliminate lead from gasoline and local policies to reduce lead emissions from municipal incinerators. Between 1970 and 1974, the number of New York children heavily poisoned by lead fell by more than 80 percent, according to data from the New York City Department of Health.
Lead levels in New York have continued to fall. One analysis in the late 1990s found that children in New York had lower lead exposure than children in many other big U.S. cities, possibly because of a 1960 policy to replace old windows. That policy, meant to reduce deaths from falls, had an unforeseen benefit -- old windows are a source of lead poisoning, said Dave Jacobs of the National Center for Healthy Housing, an advocacy group that is publicizing Nevin's work. Nevin's research was not funded by the group.
The later drop in violent crime was dramatic. In 1990, 31 New Yorkers out of every 100,000 were murdered. In 2004, the rate was 7 per 100,000 -- lower than in most big cities. The lead theory also may explain why crime fell broadly across the United States in the 1990s, not just in New York.
The centerpiece of Nevin's research is an analysis of crime rates and lead poisoning levels across a century. The United States has had two spikes of lead poisoning: one at the turn of the 20th century, linked to lead in household paint, and one after World War II, when the use of leaded gasoline increased sharply. Both times, the violent crime rate went up and down in concert, with the violent crime peaks coming two decades after the lead poisoning peaks.
Other evidence has accumulated in recent years that lead is a neurotoxin that causes impulsivity and aggression, but these studies have also drawn little attention. In 2001, sociologist Paul B. Stretesky and criminologist Michael Lynch showed that U.S. counties with high lead levels had four times the murder rate of counties with low lead levels, after controlling for multiple environmental and socioeconomic factors.
In 2002, Herbert Needleman, a psychiatrist at the University of Pittsburgh, compared lead levels of 194 adolescents arrested in Pittsburgh with lead levels of 146 high school adolescents: The arrested youths had lead levels that were four times higher.
"Impulsivity means you ignore the consequences of what you do," said Needleman, one of the country's foremost experts on lead poisoning, explaining why Nevin's theory is plausible. Lead decreases the ability to tell yourself, "If I do this, I will go to jail."
Nevin's work has been published mainly in the peer-reviewed journal Environmental Research. Within the field of neurotoxicology, Nevin's findings are unsurprising, said Ellen Silbergeld, professor of environmental health sciences at Johns Hopkins University and the editor of Environmental Research.
"There is a strong literature on lead and sociopathic behavior among adolescents and young adults with a previous history of lead exposure," she said.
Two new studies by criminologists Richard Rosenfeld and Steven F. Messner have looked at Giuliani's policing policies. They found that the mayor's zero-tolerance approach to crime was responsible for 10 percent, maybe 20 percent, at most, of the decline in violent crime in New York City.
Nevin acknowledges that crime rates are rising in some parts of the United States after years of decline, but he points out that crime is falling in other places and is still low overall by historical measures. Also, the biggest reductions in lead poisoning took place by the mid-1980s, which may explain why reductions in crime might have tapered off by 2005. Lastly, he argues that older, recidivist offenders -- who were exposed to lead as toddlers three or four decades ago -- are increasingly accounting for much of the violent crime.
Nevin's finding may even account for phenomena he did not set out to address. His theory addresses why rates of violent crime among black adolescents from inner-city neighborhoods have declined faster than the overall crime rate -- lead amelioration programs had the biggest impact on the urban poor. Children in inner-city neighborhoods were the ones most likely to be poisoned by lead, because they were more likely to live in substandard housing that had lead paint and because public housing projects were often situated near highways.
Chicago's Robert Taylor Homes, for example, were built over the Dan Ryan Expressway, with 150,000 cars going by each day. Eighteen years after the project opened in 1962, one study found that its residents were 22 times more likely to be murderers than people living elsewhere in Chicago.
Nevin's finding implies a double tragedy for America's inner cities: Thousands of children in these neighborhoods were poisoned by lead in the first three quarters of the last century. Large numbers of them then became the targets, in the last quarter, of Giuliani-style law enforcement policies.