Needleman, HL, JA Riess, MJ Tobin, GE Biesecker and JB Greenhouse. 1996. Bone lead levels and delinquent behavior. Journal of the American Medical Association 275:363-369.

Needleman et al. report that as children age, those with high lead levels show an increased likelihood of engaging in agressive and delinquent behaviors. They conclude that lead exposure is associated with an increased risk for antisocial and delinquent behaviors, and that "environmental lead exposure, a preventable occurrence, should be included when considering the many factors contributing to delinquent behavior."

What did they do? Needleman et al. recruited 301 children from the Pittsburgh Youth Study (PSY), a prospective, longitudinal study of the developmental course of delinquency. Needleman's subjects were chosen from a larger pool of PSY participants to give Needleman's team a balanced mixture boys at high and low risk to delinquency. The PSY had begun tracking these boys in first grade and obtained, at different ages, a series of different measurements of delinquency through interviews with teachers, parents and the boys themselves.

Needleman and his colleagues measured bone lead levels in the boys at age 12 and then examined the relationship between lead levels and behavioral parameters, not only delinquency but also the ability to maintain attention, neurobehavioral function (things like pattern recognition, the ability to learn and recall associations, etc.) and intelligence. In their analysis they used PSY-obtained estimates of delinquency of the boys at age 7 and again at age 11.

To avoid being misled by other familial and social factors, they evaluated a series of other parameters, including maternal intelligence, socioeconomic status and quality of child-rearing.

What did they find? At age 7, relationship between bone lead and the PSY-administered assessments of delinquency showed only borderline associations between delinquency and lead: parent interviews revealed no association while teacher interviews revealed weak associations, not quite reaching statistical significance.

By age 11, however, both parent and teacher interviews revealed significant links between several scores related to delinquency and bone lead levels. Controlling for potentially confounding variables, the parental estimates of aggressive and delinquent behavior were significantly associated with bone lead levels, and the teacher assessments even more so.

Importantly, the lead levels measured in these boys were too low too cause any overt signs of lead toxicity.

And over time, from ages 7 to 11, the boys with relatively higher lead levels were more likely to obtain worse deliquency scores.

Another important finding came from a closer look at their measurements of intelligence and its relationship to lead levels: at elevated but still relatively low levels of lead exposure, social rearing factors appeared to be more important than lead on IQ. This suggests that good parenting can ameliorate at least some degree of the impact of lead poisoning, at least at relatively low levels.

What does it mean? This work establishes the importance of lead as a risk factor for anti-social, delinquent and aggressive behavior, at lead levels below those necessary to cause overt lead poisoning. Indeed, in 1982 16% of American children had lead levels that, according to this study, would be associated with an increased risk of delinquency. While that percentage has declined significantly over the past two decades, many thousands of children, especially in urban minority populations, still have lead levels that put them at risk.

According to Needleman et al.: "The role of brain damage due to neurotoxins in eliciting antisocial behavior has, with the exception of alcohol, been largely ignored. The convergent findings in this report... suggest that altered social behavior may be among the earliest expressions of lead toxicity. These data argue that environmental lead exposure, a preventable occurrence, should be included when considering the many factors contributing to delinquent behavior."