Capital Punishment Works (Article from the Wall Street Journal)


November 2, 2007; Page A13

Recent high-profile events have reopened the debate about the value of capital punishment in a just society. This is an important discussion, because the taking of a human life is always a serious matter.

Most commentators who oppose capital punishment assert that an execution has no deterrent effect on future crimes. Recent evidence, however, suggests that the death penalty, when carried out, has an enormous deterrent effect on the number of murders. More precisely, our recent research shows that each execution carried out is correlated with about 74 fewer murders the following year.

For any society concerned about human life, that type of evidence is something that should be taken very seriously.

The study examined the relationship between the number of executions and the number of murders in the U.S. for the 26-year period from 1979 to 2004, using data from publicly available FBI sources. The chart nearby shows the number of executions and murders by year. There seems to be an obvious negative correlation in that when executions increase, murders decrease, and when executions decrease, murders increase.

In the early 1980s, the return of the death penalty was associated with a drop in the number of murders. In the mid-to-late 1980s, when the number of executions stabilized at about 20 per year, the number of murders increased. Throughout the 1990s, our society increased the number of executions, and the number of murders plummeted. Since 2001, there has been a decline in executions and an increase in murders.

It is possible that this correlated relationship could be mere coincidence, so we did a regression analysis on the 26-year relationship. The association was significant at the .00005 level, which meant the odds against the pattern being simply a random happening are about 18,000 to one. Further analysis revealed that each execution seems to be associated with 71 fewer murders in the year the execution took place.

While it is clear that the number of murders is inversely correlated to the number of executions, it is dangerous to infer causal relationships through correlative data. Causation can be a two-way street, but not in the case of capital punishment. It may be logical that more executions could lead to fewer murders, but it is not at all logical that fewer murders could cause more executions.

A second difficulty with strong correlative data is that of timing. Causes should come before effects, so we correlated each year's executions to the following year's murders and found the results to be even more dramatic. The association was significant at the .00003 level, which meant the odds against the random happening are longer than 34,000 to one. Each execution was associated with 74 fewer murders the following year.

Die-hard campaigners against capital punishment could argue that there might be yet a third variable, such as a stronger police presence or a population shift to urban areas, related to each of the other two variables. Such a variable might exist, but until it can be identified, Occam's razor suggests the simplest solution is probably the actual solution. We know that, for whatever reason, there is a simple but dramatic relationship between the number of executions carried out and a corresponding reduction in the number of murders.

The conclusion that each execution carried out is associated with the saving of dozens of innocent lives creates an extraordinarily difficult moral dilemma for those who campaign against the death penalty. Until now, those activists could look into the eyes of a convicted killer, hear his or her sad story, work tirelessly to set aside the execution and, with that goal accomplished, feel good about themselves for having "saved a life." These data suggest that the moral equation is not nearly that simplistic.

It now seems that the proper question to ask goes far beyond the obvious one of "do we save the life of this convicted criminal?" The more proper question seems to be "do we save this particular life, at a cost of the lives of dozens of future murder victims?" That is a much more difficult moral dilemma, which deserves wide discussion in a free society.

Mr. Adler is a professor of marketing and Mr. Summers is a professor of quantitative methods at Pepper dine University.