In upholding Oklahoma's use of a controversial lethal injection drug on Monday, Justice Antonin Scalia wrote that it seems "very likely" to him that the death penalty deters crime, and he cherry-picked several studies in his defense. But what seems "very likely" to Scalia apparently doesn't seem so likely to criminologists and other experts who have studied this issue.

The Death Penalty Information Center, one of the top nonpartisan sources for information about capital punishment, summarized a 2009 survey in which a large majority of criminologists said the death penalty isn't proven to deter homicides:

Eighty-eight percent of the country's top criminologists do not believe the death penalty acts as a deterrent to homicide, according to a new study published in the Journal of Criminal Law and Criminology and authored by Professor Michael Radelet, Chair of the Department of Sociology at the University of Colorado-Boulder, and Traci Lacock, also at Boulder. Similarly, 87% of the expert criminologists believe that abolition of the death penalty would not have any significant effect on murder rates. In addition, 75% of the respondents agree that "debates about the death penalty distract Congress and state legislatures from focusing on real solutions to crime problems." The survey relied on questionnaires completed by the most pre-eminent criminologists in the country, including Fellows in the American Society of Criminology; winners of the American Society of Criminology's prestigious Southerland Award; and recent presidents of the American Society of Criminology. Respondents were not asked for their personal opinion about the death penalty, but instead to answer on the basis of their understandings of the empirical research.

Part of the issue here is that the research on the death penalty's deterrent effect — including the studies that Scalia cited — is, frankly, terrible, because it's so difficult to pull out other mitigating factors that might contribute to crime. We know, for example, that states without the death penalty tend to have lower murder rates than those with the death penalty. But how much of that is related to the death penalty, or the numerous other contributors to crime and homicide rates, such as socioeconomic issues or even the amount of lead in gasoline?

Still, the overall body of research suggests there is no deterrent effect. A February 2015 review of the research by the Brennan Center for Justice found no evidence that the death penalty had an impact on crime in the 1990s and 2000s, and it concluded that the studies that suggested there was a deterrent effect were methodologically weak.

Why doesn't the death penalty pose a deterrent effect? One would think that a would-be killer would at least consider the possibility that he may be executed. But the Brennan Center for Justice report suggested that this misunderstand the thinking of most killers:

[I]t is debatable whether an individual even engages in such objective calculations before committing a crime. Much psychological and sociological research suggests that many criminal acts are crimes of passion or committed in a heated moment based only on immediate circumstances, and thus potential offenders may not consider or weigh longer-term possibilities of punishment and capture, including the possibility of capital punishment.

So Scalia may think it's "very likely" that the death penalty deters crime, and he may be able to find a few studies that suggest as much. But the criminologists and experts who have looked at the overall body of evidence have come to starkly different conclusions.