The steep drop in crime in America is one of the most noteworthy sociological trends of the last twenty years. What astonishing is that, although the murder rate has fallen by more than 50 percent in many cities, we still don't know why. Part of the mystery, of course, is that the causes are plural: there are many reasons why our cities are getting so much safer. In a recent essay, the political scientist James Q. Wilson outlines several possible explanations for the falling crime rates, from better policing to a shrinking market for crack cocaine. He also references one of the hypotheses that I'm most interested in, which is that reductions in exposure to lead gas and paint have reduced levels of aggression and improved impulse control in young men:

For decades, doctors have known that children with lots of lead in their blood are much more likely to be aggressive, violent and delinquent. In 1974, the Environmental Protection Agency required oil companies to stop putting lead in gasoline. At the same time, lead in paint was banned for any new home (though old buildings still have lead paint, which children can absorb). Tests have shown that the amount of lead in Americans' blood fell by four-fifths between 1975 and 1991. A 2007 study by the economist Jessica Wolpaw Reyes contended that the reduction in gasoline lead produced more than half of the decline in violent crime during the 1990s in the U.S. and might bring about greater declines in the future. Another economist, Rick Nevin, has made the same argument for other nations.

In recent years, neuroscientists have made important progress in identifying the precise mechanisms by which lead exposure reduces impulse control. Here, for instance, is a recent PLOS study from the Cincinnati Lead Study, in which the blood lead level of babies born in poor areas of Cincinnati were repeatedly measured between 1979 and 1984. Twenty years later, the researchers tracked down these subjects and put them in MRI machines, allowing them to measure the brain volume of participants. The researchers found that exposure to lead as a child was linked with a significant loss of brain volume in adulthood, particularly in men. Furthermore, there was a “dose-response” effect, in which the greatest brain volume loss was seen in participants with the greatest lead exposure. What's especially tragic is that the loss of volume was concentrated in the prefrontal cortex, a part of the brain closely associated with executive function and impulse control. Here are the scientists:

Childhood lead exposure is associated with region-specific reductions in adult gray matter volume. Affected regions include the portions of the prefrontal cortex and ACC responsible for executive functions, mood regulation, and decision-making. These neuroanatomical findings were more pronounced for males, suggesting that lead-related atrophic changes have a disparate impact across sexes. This analysis suggests that adverse cognitive and behavioral outcomes may be related to lead's effect on brain development producing persistent alterations in structure.

The degradation of the prefrontal cortex by lead exposure also helps explain the relationship between blood lead levels and IQ scores. According to one 2003 estimate, published in The New England Journal of Medicine, blood lead levels below the supposedly "safe" limit of 10 micrograms per deciliter still produced a reduction in IQ of around 7 points. (Approximately 1 in 50 American children has lead levels above that threshold.) That's a big cognitive loss for millions of kids.

The reason the prefrontal cortex is such an essential component of intelligence returns us to its most important function, which is controlling the spotlight of attention and regulating the short list of thoughts in working memory. (These are the thoughts we're actually thinking about.) Although we typically assume that attention and working memory are intellectual skills - they're essential for academic success and school smarts - it has become increasingly clear that they also help us regulate our emotions. Look, for instance, at Walter Mischel's classic marshmallow task, which I wrote about in The New Yorker a few years ago. The experiment involved offering four year old children a choice between eating one marshmallow right away or, if they could wait fifteen minutes, the chance to eat two marshmallows. While every kid wanted that second sweet treat, the vast majority failed to resist the temptation right in front of them. However, about 20 percent of preschoolers were able to successfully delay gratification, restraining their impulses and exerting self-control. (A follow-up study demonstrated that these kids had fewer temper problems, more friends and much higher SAT scores as high-school seniors.) So what allowed these four year olds to resist the allure of the marshmallow? The secret was attention. Here's an excerpt from the article:

Mischel’s conclusion, based on hundreds of hours of observation, was that the crucial skill was the “strategic allocation of attention.” Instead of getting obsessed with the marshmallow—the “hot stimulus”—the patient children distracted themselves by covering their eyes, pretending to play hide-and-seek underneath the desk, or singing songs from “Sesame Street.” Their desire wasn’t defeated—it was merely forgotten. “If you’re thinking about the marshmallow and how delicious it is, then you’re going to eat it,” Mischel says. “The key is to avoid thinking about it in the first place.”

Furthermore, the scientists are now doing brain scans on the original marshmallow subjects - the four year olds are now in their forties - as they try to better understand the substrate of self-control. In essence, they want to decipher the cortical persistent differences between high-delayers and low-delayers. Most of the relevant regions they've identified so far are in the frontal cortex, and include areas such as the dorsolateral prefrontal cortex, the anterior prefrontal cortex, the anterior cingulate, and the right and left inferior frontal gyri. (Needless to say, these are also many of the brain regions undermined by lead exposure.) In general, high-delayers show higher levels of activation in these areas when performing tasks requiring the allocation of attention, which is why they also perform at a higher level.

What Mischel's data demonstrates is that attention isn't just about information. Instead, it's also what allows us to blunt the urges of our errant emotions, allowing us to look past the desire to stuff that yummy marshmallow into our mouth. While we can't always control what we feel - many of our urges are ancient drives, embedded deep in the brain - we can control the amount of attention we pay to our feelings. When faced with a tempting treat, we can look away.

This returns us to aggression. Let's say you're being teased by a bully at school. You can feel your anger rising; the hot emotion is vibrating in your veins. It would feel so good to punch that bully in the face, to vent your frustration with a fight. However, you also know that such violence will get you suspended from school, which is why throwing a punch is not a good idea. If you can't strategically allocate your attention - and this is a skill that requires a solid prefrontal cortex - then you're not going to be able to resist your anger. You're going to get in a fight and get suspended. However, if you can properly look past this negative emotion - perhaps by counting to ten, or just walking away, or finding something else to think about - then your anger will subside. The hot feeling has been cooled off.

The tragedy of lead exposure is that it undermines one of the most essential mental skills we can give our kids, which is the ability to control what they're thinking about. While the unconscious will always be full of impulses we can't prevent, and the world will always be full of dangerous temptations, we don't have to give in. We can choose to direct the spotlight of attention elsewhere, so that instead of thinking about the marshmallow we're thinking about Sesame Street, or instead of thinking about our anger we're counting to ten. And so there is no fight. We walk away.