This won't come as big news to readers of this blog, but it's worth posting just for the headline alone:
A new study calculates that exposure to car exhaust from leaded gas during childhood stole a collective 824 million IQ points from more than 170 million Americans alive today, about half the population of the United States.
....The researchers calculated that at its worst, people born in the mid-to-late 1960s may have lost up to six IQ points, and children registering the highest levels of lead in their blood, eight times the current minimum level to initiate clinical concern, fared even worse, potentially losing more than seven IQ points on average.
Dropping a few IQ points may seem negligible, but the authors note that these changes are dramatic enough to potentially shift people with below-average cognitive ability (IQ score less than 85) to being classified as having an intellectual disability (IQ score below 70).
The effect of lead on violent crime is a relatively new finding and it's still controversial. The effect of lead on IQ isn't. It's one of the oldest and most widely accepted findings in the literature. There's not much question that leaded gasoline reduced human intelligence worldwide among those born around 1960-1990 or so.
And the researchers are right about the effect, which is similar to the effect on violent crime. For those in the average IQ zone (or the average violence zone) the loss of a few points of IQ isn't that big deal. But the effect of lead is at its peak in poor communities and urban cores, where more people already have below-average IQ and above-average propensities for violence. A difference of six points is the difference between, say, average IQs of 90 and 84. Or, even worse, between average IQs of 80 and 74, with similar levels of change in violent behavior. Those are gigantic differences.
POSTSCRIPT: In a stunning abuse of significant digits, the authors of the paper actually estimated a loss of 824,097,690 IQ points. Uh huh.