There are some questions that social scientists should be able to answer. Either executing people cuts the homicide rate or it does not. Or perhaps it does under certain conditions and not others. In any case, the data are readily available and researchers should be able to answer the question. Of course, this would not resolve the ethical issues surrounding the question, but that is another matter.
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Models that find deterrence effects of capital punishment often rely on rather bizarre specifications. In a rigorous and comprehensive review Cameron (1994, 214) observed that, �What emerges most strongly from this review is that obtaining a significant deterrent effect of executions seems to depend on adding a set of data with no executions to the time series and including an executing/non-executing dummy in the cross-section analysis . . . there is no clear justification for the latter practice.�
In less technical language, the researchers included a set of years when there were no executions, then introduced a control variable to eliminate the nonexistent variance. The other day upon the stair, they saw some variance that wasn�t there. It wasn�t there again today, thank goodness their model scared it away. Not all the studies rely on this particular maneuver, but they all depend on techniques that demand too much from the available data.