Why has crime declined?

We don't know exactly why, but we're pretty sure that high incarceration rates didn't do it. [Link]

But if it was not incarceration, then what did cause the crime decline?

There is no shortage of candidates. Every year, it seems, a new study advances a novel explanation. Levitt attributes about half the crime drop to the legalization of abortion. Amherst economist Jessica Reyes attributes about half the violent crime drop to the unleading of gasoline after the Clean Air Act. Berkeley law professor Franklin Zimring credits the police as the central cause. All three theories likely played some role.

Instead of a single, dominant cause, our research points to a vast web of factors, often complex, often interacting, and some unexpected. Of the theories we examined, we found the following factors had some effect on bringing down crime: a growth in income (5 to 10 percent), changes in alcohol consumption (5 to 10 percent), the aging population (0 to 5 percent), and decreased unemployment (0 to 3 percent). Policing also played a role, with increased numbers of police in the 1990s reducing crime (0 to 10 percent) and the introduction of CompStat having an even larger effect (5 to 15 percent).

But none is solely, or even largely, responsible for the crime drop. Unfortunately, we could not fully test a few theories, as the data did not exist at the detailed level we needed for our analysis. For those, we analyzed past research, finding that inflation and consumer confidence (individuals’ belief about the strength of the economy) probably had some effect on crime. The legalization of abortion and unleading of gasoline may also have played some role.

In aggregate, the fourteen factors we identified can explain some of the drop in crime in the 1990s. But even adding all of them together fails to explain the majority of the decrease.