Some proponents of capital punishment maintain that social science is incapable of determining the effectiveness of capital punishment because the data is rough and incomplete and because social science lacks a theory adequate enough to interpret the data. Arguments based on common sense, they contend, are enough to prove that capital punishment is effective.

The most powerful argument for the deterrent effect of the death penalty comes from the commonsense notion that people fear death more than life in prison. “What is feared most deters most,” says Ernest van den Haag, a professor at Fordham University and a noted proponent of capital punishment. Once in prison, virtually all convicted murderers seek to avoid execution by appealing to reduce their sentence to life in prison. To van den Haag, this is evidence that the death penalty is feared more, and therefore deters more, than a life sentence. Moreover, he argues that even though social science may not be able to prove conclusively that the death penalty deters murder (at least in statistically significant amounts), capital punishment has surely prevented some murders. Many believe this reason enough to use it.

Proponents of capital punishment also contend that the effects of a death sentence are diluted when the execution is not carried out in a reasonable period of time after sentencing. It currently takes an average of ten years from conviction to execution because prisoners abuse the writ of habeas corpus, which guarantees appeals of sentences and convictions in state criminal cases. Critics contend that this delay eliminates the cause-and-effect relationship between crime and punishment that is necessary if punishment is to deter future crimes. Some argue that if the appeals process were reformed, the deterrent effect of capital punishment would be more evident and provable.