For early childhood exposure, the risk is 1.30, but the sample size was higher, leading to a tighter confidence interval of 1.03-1.64. This range indicates it’s possible that the effect is as little as a 3% increase in violent crime arrests, but this is still statistically significant.
For 6-year-olds, it’s a much more significant 1.48 (confidence interval 1.15-1.89). It seems unusual to me that lead would have such a more profound effect the older the child gets, but I need to look into it further.
The correlation is across different states and countries who stopped using leaded gasoline at different times. Violent crime peaks around 20 years after lead pollution peaks. The crime rates in big and small cities in the US, once wildly different, have now converged, also some 20 years after the phase-out.
This large effect probably also means that the other toxins from fossil fuels could also be resulting in behavioral change that goes along with the health damage and death. Burning 12 billion tons per year of oil and coal throws a toxic chemical cocktail into the air, water and soil.
There are at least three independent strands of evidence linking lead to violent crime:
Ecological studies. These look at correlations between lead exposure and crime rates at a population level. There are now multiple rigorous studies using different methodologies that demonstrate this correlation at the city level, the state level, the national level, and in different countries at different times.
Longitudinal studies. A University of Cincinnati team began following a group of children starting in the early 80s. Every six months they measured lead levels in their blood. At age 7, kids with higher lead levels were doing worse in school. At age 17 they were more heavily involved in juvenile delinquency. At age 27 they had higher arrest rates for violent crimes.
Imaging studies. The Cincinnati team recently did a series of MRI scans of their subjects and found that participants with higher childhood lead levels had permanent damage to areas of the brain that are responsible for things like impulse control, judgment, and emotional regulation. We've long known that lead poisoning at high levels makes you more aggressive and prone to violence, and this study strongly suggests that the same thing is true even at moderate levels.
A second study found that high exposure to lead during childhood was linked to a permanent loss of gray matter in the prefrontal cortex—a part of the brain associated with aggression control as well as what psychologists call "executive functions": emotional regulation, impulse control, attention, verbal reasoning, and mental flexibility. One way to understand this, says Kim Cecil, another member of the Cincinnati team, is that lead affects precisely the areas of the brain "that make us most human."
So lead is a double whammy: It impairs specific parts of the brain responsible for executive functions and it impairs the communication channels between these parts of the brain. For children like the ones in the Cincinnati study, who were mostly inner-city kids with plenty of strikes against them already, lead exposure was, in Cecil's words, an "additional kick in the gut." And one more thing: Although both sexes are affected by lead, the neurological impact turns out to be greater among boys than girls.
Other recent studies link even minuscule blood lead levels with attention deficit/hyperactivity disorder. Even at concentrations well below those usually considered safe—levels still common today—lead increases the odds of kids developing ADHD.
In other words, as Reyes summarized the evidence in her paper, even moderately high levels of lead exposure are associated with aggressivity, impulsivity, ADHD, and lower IQ. And right there, you've practically defined the profile of a violent young offender.
It's difficult to put firm numbers to the costs and benefits of lead abatement. But for a rough idea, let's start with the two biggest costs. Nevin estimates that there are perhaps 16 million pre-1960 houses with lead-painted windows, and replacing them all would cost something like $10 billion per year over 20 years. Soil cleanup in the hardest-hit urban neighborhoods is tougher to get a handle on, with estimates ranging from $2 to $36 per square foot. A rough extrapolation from Mielke's estimate to clean up New Orleans suggests that a nationwide program might cost another $10 billion per year.
So in round numbers that's about $20 billion per year for two decades. But the benefits would be huge. Let's just take a look at the two biggest ones. By Mielke and Zahran's estimates, if we adopted the soil standard of a country like Norway (roughly 100 ppm or less), it would bring about $30 billion in annual returns from the cognitive benefits alone (higher IQs, and the resulting higher lifetime earnings). Cleaning up old windows might double this. And violent crime reduction would be an even bigger benefit. Estimates here are even more difficult, but Mark Kleiman suggests that a 10 percent drop in crime—a goal that seems reasonable if we get serious about cleaning up the last of our lead problem—could produce benefits as high as $150 billion per year.
Put this all together and the benefits of lead cleanup could be in the neighborhood of $200 billion per year. In other words, an annual investment of $20 billion for 20 years could produce returns of 10-to-1 every single year for decades to come.