A Mountain of Lies

David Brooks writes in the NYTimes:
If you want to deter crime, it seems that you’d want to lengthen prison sentences so that criminals would face steeper costs for breaking the law. In fact, a mountain of research shows that increases in prison terms have done nothing to deter crime. 

To quote the brilliant statistician John Lott:

This distrust of prison reducing crime is not new, but many have a hard time believing the simplest rule of economics: if you make something more costly, people do less of it. People accept that this principle applies to what we buy in grocery stores, but not to “bad” things that people might do.

So how plausible is deterrence? Let us take a couple examples from sports.

When college basketball’s Atlantic Coast Conference increased the number of referees per game from two to three in 1978, the number of fouls dropped by 34 percent. Why? Basketball players fouled less often because they were more likely to get caught. In fact, the actual decline in fouling was probably even larger, since fouls that may have gone unnoticed by two referees were more likely to be caught when there were three officials.

Baseball players respond no differently. The American League has more batters hit by pitchers than the National League, but this difference only occurred after 1973, when the American League removed its pitchers from the batting lineup in favor of designated hitters. Since American League pitchers no longer worried that they themselves would be hit in retaliation if they hit an opposing batter, they began throwing more beanballs.

…

Given these results, is it really difficult to believe, as the JFA Institute report claims, that the number of prisoners increased while crime rates fell? Is there really anything that makes criminals immune to these same forces?

A large number of studies indicate that the more certain the punishment, the fewer the crimes committed (for a survey click here.) Arrest rates of criminals are usually the single most important factor in reducing every type of crime. The death penalty may get the most media attention, as it deserves, but everyday police work is really important in making neighborhoods safer. Changes in the arrest rate account for about 16 to 18 percent of the large drop in the murder rate during the 1990s. Conviction rates explain another 12 percent.

By comparison, the death penalty execution rate accounts for about 12 to 14 percent of the overall drop in murders.

Prison stops crime in two ways: deterrence and incapacitation. The JFA Institute report misses both points. A longer prison term deters some would-be criminals from committing crimes to begin with. For those criminals who are not stopped by the threat of prison, at least they are taken off the streets and locked up, preventing them from committing yet more crime.

Longer prison sentences explain at least another 12 percent of the drop in murder rates. Why is it “at least”? Good data simply isn't available. It’s surprisingly difficult to measure how long criminals actually end up being in prison. The length of a criminal’s sentence is often much longer than the actual time served. Furthermore, the time that is served varies widely, even for a single type of crime, depending on a suspect’s criminal history and the severity of the offense.