By now, just about everybody knows about the two competing hypotheses that attempt to explain the drop in crime in the late 1990′s. There’s the “broken windows” theory, which assumes that crime is contextual, and that cracking down on the small misdemeanors (like public drunkenness, loitering and graffiti) eliminates the conditions that encourage felonies. And then there’s the Freakonomics theory, which is that the legalization of abortion in 1973 reduced the number of unwanted births, which led, a generation later, to a reduction in the number of criminals roaming the streets.
But now there’s a new hypothesis:
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.”
Through much of the 20th century, lead in U.S. paint and gasoline fumes poisoned toddlers as they put contaminated hands in their mouths. The consequences on crime, Nevin found, occurred when poisoning victims became adolescents. Nevin does not say that lead is the only factor behind crime, but he says it is the biggest factor.
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.”
Obviously, crime is an extremely complex social phenomenon, and it’s ridiculous to try to associate a temporary drop in criminal activity with any single explanation. I’m sure all three phenomenon – the reduction in lead exposure, the legalization of abortion and stricter policing – played some role in the drop in crime.
That said, there are other extremely important reasons to reduce lead exposure.