A drop in the exposure to lead among children in their preschool years reduces antisocial behavior and may be a factor behind the decline in crime over the past few decades, according to researchers from Princeton University and Brown University.

Princeton University economics professor Janet Currie, said in a statement:

Our results support the hypothesis that reductions in blood lead levels may have been responsible for a significant part of the observed decrease in antisocial behavior among youths and young adults in recent decades.

Currie and Anna Aizer, a professor of economics and public policy at Brown who did postdoctoral work at Princeton’s Center for Health and Wellbeing, based their study on data covering about 120,000 children born in Rhode Island. The study appeared as a working paper on the National Bureau of Economic Research website.

The report said people exposed to lead as young children up to six years old are more likely to have poor thinking skills and impulse control, to have trouble paying attention and to behave aggressively. These traits can lead to antisocial or criminal behavior in adults. Studies seeking links between adult crime and early childhood lead exposure have suggested the drop in lead exposure could explain up to 90% of the downward trend in U.S. crime that started in the mid-1990s.

The researchers examined children born in Rhode Island from 1990, which was shortly after leaded gasoline was phased out, until 2014. Rhode Island was chosen, they said, because the state took very aggressive action in screening children for lead in their blood. The researchers accessed Rhode Island Department of Health blood lead level tests for preschool children conducted from 1994 to 2014. They linked those records to school suspension records beginning in the 2007-08 school year, as well as to juvenile detention records beginning in 2004.

The researchers found that among boys, a one-unit increase in blood lead levels raised the probability of incarceration by 27% to 74%. Because few juveniles, and almost no girls, ever experience incarceration, estimates of lead’s effect on incarceration were less accurate.

“Children who have been suspended are 10 times more likely to be involved in criminal activity as adults,” Currie said. Young people who are incarcerated for even a short period are less likely to graduate from high school and more likely to commit crimes as adults.

Lead was banned from house paint in 1976, and leaded gasoline was phased out between 1979 and 1986.

The researchers suggested other explanations for the crime drop. Falling crime rates have been tied to greater availability of abortions, better police methods, the increase in the prison population and the decline of the crack-cocaine epidemic. These developments occurred around the same time and make it difficult to distinguish their effects from one another.

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