Capital punishment clearly increases the risk to criminals of engaging in various crimes, especially murder. But does this increased risk affect criminals’ behavior? Last week the academic debate erupted in the media with an Associated Press article headlined "Studies: Death Penalty Discourages Crime,” but even this recognition downplays the general consensus on the findings.

The media is a bit Johnny-come-lately in recognizing all the research that has been done on the death penalty over the last decade, with nine of the 12 refereed academic studies by economists finding that the death penalty saves lives.

Some academics are yet to be convinced and argue that the risk of a criminal being executed for murder is so remote that, “It is hard to believe that fear of execution would be a driving force in a rational criminal’s calculus in modern America.”

Yet, before trying to answer whether this risk to criminals is significant, let’s first consider how another group that faces similar dangers reacts to the risk of death.

Academics classify being a police officer as an “extremely dangerous” job. In 2005, 55 police officers were murdered on the job, while another 67 were accidentally killed. With nearly 700,000 full-time, sworn law enforcement officers in the United States, the murder rate of police officers comes to 1 in 12,500, a ratio that jumps to 1 in 5,600 when we include accidental deaths.

Police officers undertake a variety of measures to reduce the dangers: They wear bullet-proof vests, develop special procedures for approaching stopped cars and in some situations officers wait for backup even when this increases the probability that a suspect will escape.

These dangers also create strain on officers’ marriages, contributing to a divorce rate that is twice that of the general population.

Officers undertake all these measures as a natural human reaction to the risk of death -- the riskier an activity, the more a person will usually avoid it or take steps to make it safer.

The risk that a violent criminal faces from execution is much greater than the risk of a police officer being killed. In 2005, there were almost 16,700 murders in the United States and 60 executions. That translates to one execution for every 278 murders. In other words, a murderer is 20 times more likely to be executed than a police officer is to be deliberately or accidentally killed on duty.

Those who argue that the death penalty has no effect on violent crime assume that the risk of execution in no way deters criminals from committing capital crimes. While criminals, just like police officers, are naturally less adverse to danger than, say, school teachers or accountants, the notion that it is irrational for them to take into account such an enormous additional risk is irrational.

But a non-trivial issue is how to define the execution rate. It actually matters a lot.

When defined as executions per murder committed, academics find that the death penalty deters murders and saves lives.

But those academics who instead define their measure as death penalty executions per person in prison find no relationship. Which is the best measure?

Clearly, we should consider the real risk to the potential murderers, and executions per murder seems to be a much more direct measure of that risk. By contrast, executions per prisoner includes all sorts of extraneous crimes in the measure.

For example, if fewer criminals were arrested and imprisoned for stealing radios from cars, executions per prisoner inexplicably implies that the risks to committing murder increases. It is not at all surprising that this strange measure implies no real link between the execution rate and murders.

There is widespread public debate over the effectiveness of the death penalty. Unfortunately, this has included some misleading reporting in the popular press. Take a widely publicized New York Times study that compared murder rates in 1998 in states with and without the death penalty. The Times concluded that capital punishment was ineffective in reducing crime, noting that “10 of the 12 states without capital punishment have homicide rates below the national average ... while half the states with the death penalty have homicide rates above the national average.”

This simple comparison really doesn’t prove anything. The 12 states without the death penalty have long enjoyed relatively low murder rates due to factors unrelated to capital punishment.

When the death penalty was suspended nationwide from 1968 to 1976, the murder rate in these 12 states still was lower than in most other states. What is much more important is that the states that reinstituted the death penalty had about a 38 percent larger drop in murder rates by 1998.

There were no executions in the United States between 1968 and 1976, a time when murder rates skyrocketed. Various explanations were put forward in the 1970s to explain the jump in violent crime.

Some claimed that the Supreme Court’s Miranda decision — mandating that suspects be read their rights during arrest — reduced criminal confessions and otherwise hindered convictions. Other theories blamed softer criminal penalties or lower arrest rates. Back in the 1970s these studies were generally inconclusive, however, due to the poor quality of the data available at the time, especially a lack of crime statistics by state.

This research was conducted as violent crime rates were plummeting while executions were rising sharply. Between 1991 and 2000, there were 9,114 fewer murders per year, while the number of executions per year rose by 71.

Generally, the studies over the last decade that examined how the murder rates in each state changed as they changed their execution rate found that each execution saved the lives of roughly 15 to 18 potential murder victims. Overall, the rise in executions during the 1990s accounts for about 12 to 14 percent of the overall drop in murders.

Of course, there are exceptions to capital punishment deterring murder. One particular kind of crime where the death penalty shows no significant deterrent effect is multiple victim public shootings. This was the conclusion of a study I performed with Bill Landes at the University of Chicago.

This exception, however, is easy to explain: The vast majority of these killers either commit suicide or are killed at the scene of the crime. The threat of legal punishment, including the death penalty, doesn’t really affect their actions since so many of these criminals expect to die in the course of their crime.

Compared to more sophisticated Europeans, Americans long have been portrayed as uneducated yokels for supporting the death penalty. And the Supreme Court has looked to guidance from other countries in justifying limits on the death penalty. But Americans have stuck to their guns, with the majority of Americans in a May 2006 Gallup poll even feeling that the death penalty should be used more frequently.

Possibly it is time to concede that everyday Americans might actually know something that some members of the Supreme Court have had a hard time understanding.

John Lott is the author of the forthcoming book "Freedomnomics" and will soon be a Senior Research Scientist at the University of Maryland.