Over the past two decades, the rate of violent crime in the U.S. and Europe has dropped dramatically. Despite increasingly sensational news stories about crime, we are in fact much less likely to become the victim of a violent crime today than we were in 1990. According to the New York Times, the city of New York had fewer murders last year than in any year since 1963, when reliable record keeping began. In 2013, there were 333 murders in the city, down from 417 in 2012 and a stunning 2,245 in 1991. Chicago, Los Angeles, and Philadelphia also had large declines in violent crime during this period, as did smaller cities across the country. In England, the 2013 murder rate was at a 33-year low, nearly 50% lower than its peak in 1995, according to a recent story in the Guardian. There is no question that the rate of violent crime is significantly lower than it was 20 years ago.

Many factors could be contributing to this change, including improvements in law enforcement, reductions in the use of crack cocaine and other drugs, economic changes, and the aging of the population. However, a study by economist Rick Nevin suggests that reductions in the crime rate can be attributed to diminishing levels of lead poisoning from exposure to leaded gasoline and lead paint—and there is a growing body of research that supports his theory.

“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,” says the Washington Post’s Shankar Vedantam.

In a recent Forbes article, science writer Alex Knapp outlines reasons that Nevin’s theory deserves attention. First, the numbers correlate almost perfectly; when a lag time of 21 years is added (to account for early childhood lead exposure in adult offenders), levels of exposure to lead from gasoline and paint track extremely closely with the U.S. homicide rate (see the graph in Nevin’s 2013 update).

Second, the correlation holds true with no exceptions. “Every country studied has shown this same strong correlation between leaded gasoline and violent crime rates,” says Knapp. “Within the United States, you can see the data at the state level. Where lead concentrations declined quickly, crime declined quickly. Where it declined slowly, crime declined slowly. The data even holds true at the neighborhood level—high lead concentrations correlate so well that you can overlay maps of crime rates over maps of lead concentrations and get an almost perfect fit.”

Third, the connection between lead poisoning and brain damage is clear. “Not only does lead degrade cognitive abilities and lower intelligence, it also degrades a person’s ability to make decisions by damaging areas of the brain responsible for emotional regulation, impulse control, attention, verbal reasoning, and mental flexibility,” says Knapp.

Nevin’s conclusions have been criticized by some, including those who are wary of the implications of linking biology to criminal behavior. In a recent interview with BBC News Magazine, Roger Matthews, a professor of criminology at the University of Kent, said, “The things that push people into crime are very different kinds of phenomena, not in the nature of their brain tissue….There has been a long history of people trying to link biology to crime—that some people have their eyes too close together, or an extra chromosome, or whatever. This stuff gets disproved and disproved. But it keeps popping up.”

What do you think about the link between lead levels and crime? Are the correlations strong enough to imply causation? What are the social implications of high lead levels in the blood? PAR wants to hear from you, so leave a comment and join the conversation!