When he was running for president in 2000, George W. Bush was often asked about the fact that as governor of Texas, he executed 152 people, more than any other governor in modern history at the time (though his successor Rick Perry has since surpassed him). Bush always responded that he believed the death penalty saves lives. In other words, his primary justification was a practical argument, not a moral argument. But the empirical evidence on the question of whether the death penalty was always fuzzy at best.
Like most death penalty opponents, I was always very skeptical of claims like Bush's (isn't that odd, how our beliefs about what is always seem to line up so neatly with our beliefs about what ought to be). Despite what you might believe from watching Law & Order, most murders aren't carefully planned so that the perpetrator can get his hands on his grandmother's fortune, giving him plenty of time to contemplate the potential consequences if he gets caught. People who kill other people tend to do it out of anger or desperation, and the idea that some significant number of them would stop themselves if they knew they might be executed if they got caught, but go ahead with the murder if they knew they'd spend the rest of their lives in jail, just doesn't make much sense.
So why haven't social scientists been able to answer this question definitively? The main problem is that there just aren't enough executions to give you the kind of healthy sample size you need. As Betsey Stevenson and Justin Wolfers tell us, the National Academy of Sciences has concluded that the existing research on the topic just can't tell us one way or the other whether the death penalty has any deterrent effect. Stevenson and Wolfers explain what this means:
Even if one accepts the possibility that the threat of death deters some would-be murderers, that doesn’t mean it’s the best way to do so. Capital punishment diverts hundreds of millions of dollars from other criminal-justice interventions that may have done more to reduce homicide rates. This important point -- there’s an opportunity cost to spending on capital punishment -- often gets overlooked.
Amid all the uncertainty, the data do allow one conclusion that the National Academy should have emphasized more strongly: The death penalty isn’t the dominant factor driving the fluctuations in the U.S. homicide rate. If it were, the homicide rate in the U.S. wouldn’t have moved in lockstep with that of Canada, even as the two countries experimented with different death-penalty regimes (see chart). Likewise, homicide rates tend to rise and fall roughly in unison across states, even as some - - such as Texas -- ramp up executions, and others have chosen not to adopt the practice (see chart).
Overall, the panel’s conclusions are a welcome corrective to a debate in which politically expedient, yet imperfect, findings have attracted greater attention than those rare moments of humility when we social scientists admit what we don’t know. Now that a widely respected authority has established the uncertainty about the deterrent effects of the death penalty, it’s time for advocates on both sides to recognize that their beliefs are the product of faith, not data.
Deterrence isn't the only practical question—as we've seen, our criminal justice system convicts lots and lots of innocent people, and that continuing problem offers a strong reason for eliminating the death penalty. But in the end, we're left with a fundamental moral divide, one we should explore. Should the state be killing people who are convicted of crimes? Does the fact that justice can be served in one way by an execution mean that it should be served in that way, as opposed to other ways (like life in prison, a punishment which is arguably far more harsh). What kind of values are expressed by executions, and are those the values we as a society want to promote? In every other advanced democracy, they've answered "no" (see here for details).
In a way, the practical claims are too easy to make. It certainly made it easy for Bush—he could just say the death penalty saves lives, and he didn't need to provide a moral justification for the execution assembly line they have in the Lone Star State. The moral claims require you to be clearer about your values. That's a debate we ought to have.