"I began with the city that was the crime capital of America," Giuliani, now a candidate for president, recently told Fox's Chris Wallace. "When I left, it was the safest large city in America. I reduced homicides by 67 percent. I reduced overall crime by 57 percent."

    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.

    "It is stunning how strong the association is," Nevin said in an interview. "Sixty-five to ninety percent or more of the substantial variation in violent crime in all these countries was explained by lead."

[...]
Nevin says his data not only explain the decline in crime in the 1990s, but the rise in crime in the 1980s and other fluctuations going back a century. His data from multiple countries, which have different abortion rates, police strategies, demographics and economic conditions, indicate that lead is the only explanation that can account for international trends.

    Because the countries phased out lead at different points, they provide a rigorous test: In each instance, the violent crime rate tracks lead poisoning levels two decades earlier.

[...]

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.

[...]

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 [draconian, zero-tolerance]law enforcement policies.

I'm not specifically endorsing this guy's theory, just pointing out that the effects of just one environmental toxin may take years to manifest, it may take several dispersal and reduction cycles in different environments to provide any convincing correlation, etc.   It would be a lot cheaper -- not to mention socially more just, ethically more sensible -- to conclude that a policy of slow poisoning (allowing industries license to emit toxins so long as they remain below the exposure level where people actually keel over on hour-to-year timescales) is a bad idea.  The precautionary principle, and all that...

The difference between theory and practise in practise ...