Exposure to lead as a child doesn't just reduce intelligence and harm mental health as an adult, it also changes personality, an enormous study has found. The results are in keeping with previous discoveries about the effects of childhood lead exposure, but their implications are profound in light of the vast widespread low-level lead poisoning of a generation.

Lead is a neurotoxin, dangerous at almost any level. There is abundant evidence it can reduce lifetime IQ scores in those exposed as children. It has also been associated with lower impulse control and increased violence. Considerable research supports the idea that the rise in crime experienced in the industrialized world from the late 60s to the 90s, and the subsequent spectacular fall, was the consequence of putting lead in gasoline and then taking it out. 

Dr Ted Schwaba, of the University of Texas, Austin, investigated whether lead exposure also influences adults' personalities on the “Big Five” personality traits. In Proceedings of the National Academy of Sciences, Schwaba used lead measurements in the childhood neighborhoods of 1.5 million people across America and Europe who took online personality tests.

“We take our personalities with us everywhere," Schwaba said in a statement. "Even a small negative effect of lead on personality traits, when you aggregate it across millions of people and all the daily decisions and behaviors that our personality influences, can have really massive effects on well-being, productivity and longevity." Moreover, Schwaba and co-authors note, modest lead-induced effects within the normal range of personality traits have only been explored in one small previous study. 

Schwaba and co-authors found less healthy personalities among adult Americans who grew up in counties with high lead levels. On its own, this finding could be confounded by factors such as differences in urban and rural cultures. However, the Clean Air Act phasing out leaded petrol was not implemented with equal speed everywhere. Schwaba found that people born after lead concentrations in their county fell reaped a benefit not seen in counties that only got lead-free petrol later.

Data from Europe, where leaded petrol was phased out later, confirmed two of Schwabas three main findings. On both continents, lead exposure translated to greater neuroticism and lower agreeableness. However, it had an opposite association on conscientiousness, throwing into question whether the effect on this trait was real.

"These three traits … make up a large part of what we would consider a mature, psychologically healthy personality and are strong predictors of our success or failure in relationships and at work," Schwaba said. They normally improve over a lifespan, but for people with high lead exposure maturity comes harder and later.

Generational stereotypes usually fail to stand up to scientific testing, but if Gen X, who suffered by far the highest lead exposure, are unusually neurotic and inclined to complain to the manager, we now know why.

"For a long time, we've known lead exposure is harmful, but each new wave of research seems to identify new ways in which lead exposure harms society," Schwaba said. Leaded petrol is gone, but Schwaba noted many lead-lined water pipes have yet to be replaced, and much topsoil remains contaminated. He noted Black children in America are twice as likely to be exposed as whites.

Schwaba's work suggests past estimates that lowering lead exposure further could be worth $1.2 trillion in the United States alone probably undersell the potential benefits.