Imagine the moments leading up to a murder. Are we to believe a person about to commit a homicide would consider whether or not his state has a death penalty as opposed to just life imprisonment without the possibility of release? Common sense suggests that lucid, rational calculations about capital punishment statutes are not among the things that would cross the mind of a would-be murderer. Yet as illogical as it sounds, new reports are asking us to accept this very line of reasoning.

The reports, including a Wall Street Journal op-ed by two professors from Pepperdine University, have revived the long-discredited argument alleging that capital punishment has a deterrent effect on homicide rates. They are certainly not precise or even consistent with each other, claiming that each execution deters anywhere between three and 74 murders.

Nevertheless, the new reports have captured widespread media attention, which in turn has accorded them a degree of unjustified legitimacy in the public arena. Detailed follow-up studies by prominent social scientists have demonstrated that the available data don't -- and because of inherent limitations probably can't -- support the conclusion that capital punishment deters murder.

The first glaring problem is the studies' failure to consider alternate explanations for declining murder rates in recent years. The possible alternatives include, among other things: the increase of life sentences without release, the improved police "clearance" rates for felonies, and the waning of acute drug epidemics such as the crack problem of the early 1990s. Any study that doesn't take into account these obvious factors can't be taken seriously.

Another problem -- inherent to any statistical study of deterrence -- is that the number of executions is small and insufficient to determine with any certainty the effect of executions on homicide rates.

Economist and law professor John Donohue of Yale University, together with economist Justin Wolfers of the University of Pennsylvania,