Last week, James Q. Wilson, writing in The Wall Street Journal, described several of the factors that have led to a sustained decrease in crime. One reason is increased incarceration:

One obvious answer is that many more people are in prison than in the past. Experts differ on the size of the effect, but I think that William Spelman and Steven Levitt have it about right in believing that greater incarceration can explain about one-quarter or more of the crime decline. Yes, many thoughtful observers think that we put too many offenders in prison for too long. For some criminals, such as low-level drug dealers and former inmates returned to prison for parole violations, that may be so. But it’s true nevertheless that when prisoners are kept off the street, they can attack only one another, not you or your family.

Wilson also raises the role, which is debated, argued by Donohue and Leavitt of abortion in reducing unwanted pregnancies. While there are some methodological questions, that might account for up to a quarter of the reduction in crime. But the largest effect is something I’ve mentioned before–lead poisoning:

There may also be a medical reason for the decline in crime. 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.

I bring this up, since we’ve known for a long time that lead poisoning can lower IQ. Recent work now links lead paint to poor academic performance:

Children who ingested even small amounts of lead performed poorly later on school tests compared with students who were never exposed to the substance, according to a new study of Connecticut students.
The Duke University study also found that black children were much more likely to have experienced lead poisoning from paint residue, dust, or other sources by age 7 than the state’s white children. Educators worry that factor might be among many contributing to Connecticut’s status as the state with the largest achievement gap between the races.
Education and public health officials called the Duke study a stark reminder that although lead poisoning cases have dropped sharply nationwide in recent years, even very low levels of exposure can irreversibly influence children’s development….
Connecticut educators worry the lead exposure among black students could be one of many factors in the achievement gap between white and minority students, and between wealthy and poor students.

Clearly, this is entirely the fault of slacker teachers and their evil unions. We have to start thinking more about the external environment in terms of educational outcomes. As I’ve said before, the failure of schools with lots of poor students might have something to do with poverty.