So we have little reason to ascribe the recent crime decline to jobs, the labor market, or consumer sentiment. The question remains: Why is crime falling?

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 right in believing that greater incarceration can explain one-quarter or more of the crime decline.

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Another possible reason for reduced crime is that potential victims may have become better at protecting themselves by equipping their homes with burglar alarms, installing extra locks on their cars, and moving into safer buildings or even safer neighborhoods.

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Policing, as City Journal readers know, has become more disciplined over the last two decades; these days, it tends to be driven by the desire to reduce crime, rather than simply to maximize arrests, and that shift has reduced crime rates. One of the most important innovations is what has been called hot-spot policing.

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There may also be a medical reason for the crime decline. For decades, doctors have known that children with lots of lead in their blood are much more likely to be aggressive, violent, and delinquent.

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Yet one more shift that has probably helped bring down crime is the decrease in heavy cocaine use in many states.

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Blacks still constitute the core of America's crime problem (see "Is the Criminal-Justice System Racist?," Spring 2008). But the African-American crime rate, too, has been falling, probably because of the same noneconomic factors behind falling crime in general: imprisonment, policing, environmental changes, and less cocaine abuse.

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John Donohue and Steven Levitt have advanced an additional explanation for the reduction in black crime: the legalization of abortion, which resulted in black children's never being born into circumstances that would have made them likelier to become criminals. I have ignored that explanation because it remains a strongly contested finding, challenged by two economists at the Federal Reserve Bank of Boston and by various academics.

At the deepest level, many of these shifts, taken together, suggest that crime in the United States is falling--even through the greatest economic downturn since the Great Depression--because of a big improvement in the culture. The cultural argument may strike some as vague, but writers have relied on it in the past to explain both the Great Depression's fall in crime and the sixties' crime explosion. In the first period, on this view, people took self-control seriously; in the second, self-expression--at society's cost--became more prevalent. It is a plausible case.

Culture creates a problem for social scientists like me, however. We do not know how to study it in a way that produces hard numbers and tested theories. Culture is the realm of novelists and biographers, not of data-driven social scientists. But we can take some comfort, perhaps, in reflecting that identifying the likely causes of the crime decline is even more important than precisely measuring it.