This piece was submitted to the Washington Post in response to their op ed A Death Penalty Puzzle -- The Murky Evidence for and Against Deterrence by Cass R. Sunstein and Justin Wolfers (Washington Post, Monday, June 30, 2008; Page A11)
Death-penalty laws cannot be evaluated like belt wearing laws
Leonard Evans, 2008-07-10
Do studies using data show that the death penalty deters murder? This recurring question refuses to go away. Different Supreme Court Justices selected different studies to support opposing opinions in recent decisions. The June 30, 2008 issue of the Post contained an Op Ed by the authors of some of the studies titled “Death Penalty Puzzle - The Murky Evidence for and Against Deterrence.” While I have no credentials in law or criminology, I do have decades of experience deciding what can and cannot be inferred from data. This is based on a career as a research scientist studying a field all too rich in reliable data, namely deaths in traffic crashes. This experience leads me to conclude with confidence that there is no murky evidence for or against deterrence. There is no evidence at all, and there never can be.
If, say, the death penalty increased or decreased the murder rate by 10%, there is simply no possibility of observing such an effect. If such an effect could be observed, then the effect of passing a belt-wearing law should be easily measured to high precision. Compared to murders, traffic deaths are far more numerous, clearly defined, and stable from year to year. The public knows that the belt law applies to everyone after a well publicized date, and that specified punishments follow soon after detected violations. It is reliably known that belts reduce the risk of death in a crash by 42%. These are all factors that greatly facilitate any analysis. Despite such overwhelming advantages, the effect of passing a belt-wearing law on traffic deaths is not observable in the majority of states (and countries) because sample sizes are too small to detect the expected changes in fatalities. Even when fatality reductions are reliably observed in large states, doubters argue that they are not due to the belt law but to other factors that changed. Things are always changing. Traffic deaths will decline in 2008 because of increased fuel costs.
To expect any observable effect after a state passes or repeals a death-penalty law is to deny mathematical and other truths. Of course an observed reduction in murders would follow if the death penalty reduced the murder rate by 70% -- but nobody makes so absurd a claim. The data available can never address whether the death penalty affects the murder rate by important yet small amounts like 10%. While ideally the quantitative effect of a public policy should be known, this is rarely possible. Why does the question of data-based evidence keep cropping up for the case of the death penalty when it is almost never mentioned for other policies?
The policy that is at the core of pedestrian safety is that pedestrians should look before crossing the road. There is not a shred of data-based evidence that it is safer to look than not to look. Likewise, there is not a shred of data-based evidence proving that sending people to prison for not paying their income taxes deters income-tax fraud, or for that matter, that punishing burglars reduces burglary.
When scientific reasoning shows that data cannot provide answers we must acknowledge this rather than confusing the issue by claims that data supports one side or the other. We must then proceed as best we can by debating using those flawed weapons of belief, philosophy, reason, and common sense.
I look before crossing the road, and consider it good public policy to vigorously encourage everyone to do likewise.