In the 1970s, crime was soaring, and American policymakers had all sorts of ideas for how to reduce it: longer sentences, more police, prison reform and more. But one of the most potent remedies was not conceived as a way to combat crime.
To clean up the environment and improve public health, the federal government banned lead in paint and gasoline. By diminishing lead, though, it reduced the harm it was doing to young brains -- harm that could push kids into delinquency. Curbing lead exposure was a big reason for the decline in violent crime that began in the 1990s.
Americans regard theirs as the land of opportunity, which makes the persistence of black poverty baffling and even exasperating. After all, the road out of permanent destitution is not hard to identify. Why do so many people refuse to take it?
Rick Santorum made this point when he ran for president in 2012. Experts, he asserted, have documented that as a rule, you have to do just three things to avoid poverty: "work, graduate from high school and get married before you have children."
There's much value in that formula. But putting it to use requires certain capacities. What research has starkly revealed is that poverty and other problems afflicting many black neighborhoods have a way of stunting the attributes needed to overcome them. There is a biology of poverty that is not easy to overcome.
One of the things you need to pull yourself up is a healthy brain. But poor people can't take that as a given. One enemy of sound mental function is lead, which seriously impairs cognitive development.
In 1995, as the Chicago Tribune's Michael Hawthorne recently reported, more than 80 percent of kids in some of Chicago's poorest areas had dangerously high lead levels. If you know the rate of lead poisoning among children younger than 6 in a Chicago neighborhood in 1995, he found, you can predict with uncanny accuracy its current rate of violent crime.
The kids whose brains were attacked by lead back then are now young adults. Many of them show the effects in lower intelligence and less self-control. Even today, poor African-American areas are unusually prone to lead poisoning.
But lead is just one of several hazards. Blacks are far likelier than other groups to live in areas of concentrated poverty. They pay a high price for that luxury.
A study this year in the journal Nature Neuroscience found that poor children, on average, actually have smaller brains than affluent ones. New York University sociologist Patrick Sharkey concludes that "the effect of being raised in a family that lives in a poor neighborhood over two consecutive generations is roughly equivalent to missing two to four years of schooling."
Kids raised in these places suffer other problems besides material deprivation. Poor parents are less likely to read to and talk with their children. Violence is far more common than in other places, and violence has invisible but severe consequences -- not just on direct victims but on other residents.
Children who feel unsafe at school, who are disproportionately black, do measurably worse academically. Those who witness shootings or suffer violent attacks may develop post-traumatic stress disorder.
Chronic violence carries hazards for the mind, as well as the body. "Simply put," writes Princeton sociologist Douglas Massey, "people who are exposed to high levels of stress over a prolonged period of time are at risk of having their brains rewired in a way that leaves them with fewer cognitive resources to work."
Having fewer cognitive resources makes it harder to do those three things Santorum recommended. Getting and keeping a job is harder -- and the job you get will pay less than it might otherwise. Completing high school is harder. Exercising self-discipline and using contraception are harder.
It may be said in response that many whites and immigrants managed to overcome humble beginnings. That's true. But most of them didn't have to grow up in places where poverty, environmental contamination and gunplay were as pervasive as they are in many urban black areas.
Some kids can triumph over all these. But the chronic onslaught of adversity ensures that many, if not most, will be tripped up.
When these young people fail, a lot of Americans will blame them for not doing simple, obvious things to improve their lives. In reality, much of their fate is beyond their control.