“Nevin’s conclusions amplify earlier studies linking lead exposure and criminal behavior, none more striking than work by Deborah W. Denno, a professor at Fordham University School of Law.
Longitudinal studies analyze the same group of individuals over a period of time, and Denno carried out one of the nation’s largest on the biological, sociological, and environmental predictors of crime. She did this using data collected on a group of 487 young black males from the time their mothers entered the hospital to give birth to age 25. In all, she weighed more than 3,000 variables over 25 years to test differing theories on crime. Her findings were published in her 1990 book, Biology and Violence.
Supported by the National Institute of Justice, an arm of the US Justice Department, Denno’s study began as her PhD dissertation at the University of Pennsylvania in 1978. She drew from data collected by the Collaborative Perinatal Project, a study of some 58,000 pregnancies in 12 US cities conducted from 1959 to 1974. Still used today as a research resource, the NIH-sponsored study included health, socioeconomic, religious, family, and employment data; eye and foot preferences; the IQs of subjects and their mothers; and serological, neurological, psychological, hearing, language and speech, and genetic data.
The 487 subjects Denno chose were all born in the Pennsylvania Hospital in Philadelphia between 1959 and 1962 and went through the city’s public-school system. Denno says the “gold mine” of data allowed her to control for hundreds of factors. Her subjects were African-American males because the number of whites was too small for inclusion in her analysis, but Denno contends that the subjects’ homogeneity strengthened the results of her study.
“We knew their life circumstances in great detail,” Denno says, “and given everything about them, their neighborhood, their low socioeconomic status and family situations—the very things sociologists and criminologists look at to explain criminal behavior—these young men should have all been committing crimes, but they weren’t. So the big question we asked was, if they all have such similar backgrounds and environments, why did some of them commit crimes and others not?”
After performing computer regression analyses on all the different factors, something unexpected emerged: “What came popping up again and again,” Denno says, “was the amazingly powerful effect lead had on crime. It totally blew me away.”
Before her study, Denno, like Nevin, thought the idea that a heavy metal could have anything to do with crime was “nonsense.”
“No one had ever looked at lead and crime,” she says. “Criminologists hadn’t even considered it.” She included blood-lead levels as a risk factor only because the data were available along with other blood studies performed in the perinatal project.
She found it “stunning” that elevated childhood lead levels emerged as a powerful predictor of antisocial behavior in her three basic categories—school disciplinary problems, delinquency, and adult criminality. Lead turned out to be the only factor out of the thousands she accounted for to have an impact in all three areas.
Childhood lead poisoning was the single greatest predictor of school disciplinary problems, which in turn were the major predictor for juvenile crime. She found anemia to be the second-leading predictor for school-discipline problems. Anemia is a common symptom of lead poisoning because lead inhibits the function of hemoglobin, vital for blood oxygen transport.
Denno found childhood lead poisoning to be the fourth-leading predictor of adult crime, but the leading predictor for adult crime was the number and seriousness of juvenile offenses. In other words, she says, lead was, directly or indirectly, the leading predisposing factor for all three categories of antisocial behavior.
“Among the most striking results we found,” she adds, “is that lead proved to be a key factor for the most violent offenders, those kids who committed homicides, rapes, and other violent crimes.”
Denno acknowledges that blood-lead tests in the 1950s and ’60s were not as sophisticated as they are today. “But as every good researcher knows,” she says, “the greater the precision for measuring a variable, the better your results will be. In other words, if there had been a more precise measurement, the lead effect would have been even stronger.”
Denno concludes, “I am very confident in my study, confident that lead predisposes people to act in an impulsive, antisocial way. But I found these results to be extraordinarily sad because it is all so preventable.”
Other studies support Denno’s and Nevin’s findings, and evidence of a link between childhood lead exposure and violent behavior is now so persuasive that the question is not whether lead contributes to violent crime but how much.” (Pekkanen, 2006)