Despite botched executions like the one Tuesday night in Oklahoma, a majority of Americans support the death penalty. Most people in favor of capital punishment believe that it is the only just penalty for some crimes, according to a Pew Research Center survey conducted in 2011. Other subjects of the survey cited other reasons, such as religious teaching.

By contrast, the question of whether executions discourage criminals from violent acts is not up to the conscience to decide. Despite extensive research on the question, criminologists have been unable to assemble a strong case that capital punishment deters crime.

"We're very hard pressed to find really strong evidence of deterrence," said Columbia Law School's Jeffrey Fagan.

States have been executing fewer and fewer people over the past 15 years. Several states have recently abolished capital punishment, and Gov. Jay Inslee (D) placed a moratorium on executions in the state of Washington in February. The execution in Oklahoma points to the problems that states that continue the practice are encountering. Meanwhile, however, rates of violent crime are still falling steadily.

Fagan pointed to New York as an example. Former Gov. George Pataki (R) reinstated capital punishment in New York in 1995, and although no prisoners were executed, the law remained in place until the New York Court of Appeals struck it down in 2004. Whether or not criminals faced the threat of death seemed to have little effect on their behavior. "New York's homicide decline has continued before the capital-punishment statute, through the capital-punishment statute, and after the capital-punishment statute," Fagan said.

Fagan and two collaborators recently compared murder rates in Hong Kong, where capital punishment was abolished in 1993, and Singapore, where a death sentence is mandatory for murder and other crimes and is typically administered within a year and a half. The researchers found little difference between the two Asian metropolises.

Another study compared the amount of violence in U.S. states with and without the death penalty and also failed to find a deterrent effect.

These studies do not prove that capital punishment does not deter criminals. There might be an effect that is invisible in these graphs because, for example, it is too small.

"We certainly can’t say there is a deterrent. We can’t say there is not either," said Marc Mauer, the executive director of The Sentencing Project, adding that the lack of evidence was itself worth considering. "I think at the very least the fact that there’s certainly no reason to believe there’s a significant deterrent effect should give pause."

Given the uncertainty about the death penalty's effect on crime, it's important to ask whether there is a deterrent effect of sentences of life without parole. After all, no one argues that those who are guilty of murder should go completely unpunished. Unfortunately, that is a question that researchers haven't been able to examine due to a lack of comprehensive data.

Still, there are reasons to doubt that criminals would change their behavior to avoid the risk of execution. Killers might not be in a state of mind to coolly evaluate their chances of being caught, tried and put to death, especially since appeals can continue for many years and only about half of those sentenced to death are eventually executed.

In fact, research suggests than criminals are mainly concerned about whether they'll be caught, not what might happen to them afterward.

"It’s the certainty of apprehension that’s been demonstrated consistently to be an effective deterrent, not the severity of the ensuing consequences," said Daniel Nagin, a professor at Carnegie Mellon University.

Nagin led a committee at the National Research Council that reviewed the evidence on executions and crime and concluded that the existing research is inconclusive. In any case, he argues, effective law enforcement is most important in preventing crime. People are more likely to break the law when they feel they can get away with it.

"The police are really at the center of the action in terms of deterrence," Nagin said.

Many homicides result from botched robberies. Good police work might make someone who is meditating murder hesitate, but just as importantly, it can discourage the kinds of criminal behaviors that become fatal when things go wrong.