Childhood exposure to leaded gasoline may be responsible for more than 150 million mental health disorders in Americans alive today. That’s the shocking takeaway from a recent study published in the Journal of Child Psychology and Psychiatry. It suggests that anyone born before 1996—when leaded gasoline for passenger vehicles was finally banned—was almost certainly exposed to harmful levels of this heavy metal as a child. As a result, huge chunks of the population are walking around with at least some level of reduced cognition, and researchers believe this may help explain some troubling trends and behaviors that plague our entire society.

Countries across the world started putting lead into gasoline in the 1920s because it helped improve engine performance in cars. In America, the practice began in 1923. To say this was a bad decision for public health would be an understatement. “Lead is toxic to almost every organ system we’ve studied,” says Aaron Reuben, a postdoctoral scholar in neuropsychology at Duke University and one of the study’s authors.

It’s uniquely harmful to the brain—especially the young brain. Kids younger than 5 who are exposed to even small amounts of lead can be left with cognitive damage that stays with them for life. With the introduction of leaded gasoline, millions of American cars began spewing a neurotoxin from their exhaust pipes, scattering this poison into the air, soil, and water for humans to ingest and inhale. And this went on for nearly 75 years. Previous research estimates that 170 million living Americans—or roughly half the current U.S. population—were exposed to high lead levels as children.

What does that do to a society?

We already know that lead exposure has been linked to lower intelligence: A previous study estimated that childhood lead exposure is responsible for an 800-million-point drop in the collective American IQ. But for this new study, researchers wondered about other symptoms of disrupted brain development—like depression and anxiety, hyperactivity, neuroticism, lack of impulse control, and low conscientiousness. In other words, they wanted to know whether the pernicious legacy of lead extended to the nation’s overall mental health, especially for people born in the 1960s and ’70s, before the phaseout began and the use of leaded gasoline use was at its highest.

Looking at historical blood data and previous studies linking lead exposure to mental health disorders, the researchers developed a points system that estimates a person’s liability to developing symptoms of mental illness. The more points one has, the higher their risk. The findings were startling: The U.S. population as a whole gained 600 million mental illness points between 1940 and 2015 from childhood lead exposure, mostly due to gasoline.

For everyone who was exposed, this shifted their cognitive health in the wrong direction. For some people who were already at high risk of mental health disorders, it was enough to push them over the edge. By the researchers’ calculations, 151 million “excess mental disorders” (including depression, anxiety, and attention deficit hyperactivity disorder) in people alive today may be attributable to this shift.

“We can see that lead shifted the curve so that everyone’s doing a little worse day to day, and that those who are at higher risk are going to go on to develop cases of diagnosable disorders that they wouldn’t otherwise have,” Reuben says.

For people whose baseline mental health risk was lower, the effects of lead exposure were likely less severe, but insidious nonetheless. According to researchers, they may be prone to low moods, struggle with impulse control, or display a general lack of conscientiousness, all traits that have been associated with lower levels of health, wealth, and happiness.

“I think of it as like a low-grade fever, or a weathering,” Reuben says. “We are just exerting a downward pull on everyone’s capacity to get along with each other, to function in a way that fulfills their goals, to feel good day to day.”

It’s even possible that the lingering effects of childhood lead exposure on personality and behavior have influenced world events and societal trends—from the gloominess that hangs over Generation X to the crime waves of the 1980s and ’90s to the worldwide mental health crisis.

“We dosed the entire planet,” Reuben says. And many countries continued using leaded gasoline long after the U.S. stopped. The last stockpile of the stuff was only used up in 2021, in Algeria. “Any social disruptions you’re seeing, any macro trends and geopolitics, are all being influenced by this—some more subtly, some less subtly. And who knows how long it will take us to disentangle,” Reuben adds.

Looking to the future, evidence suggests that kids who were exposed to lead will experience higher rates of dementia as they age. So we’ll be living with the effects of leaded gasoline for a long time yet. But it’s not like we’ve stopped using lead altogether. Global lead output is expected to grow this year to nearly 5 million metric tons. It’s still a component in jet fuel, and millions of Americans are at risk of exposure through lead water pipes and old paint.

“We are mining, using, and producing more lead today than at any point in human history,” Reuben says. “I am extremely skeptical that all the lead we’re using now is not going to find its way into our food, our water, our bodies over time. So we need to devote more and more resources to identifying where the legacy lead is and taking it out.”