Sunstone, I'm surprised that nobody has pointed out the correlation/causation error in this argument yet. Of
course there's a higher percentage of theist criminals in prison; prisons historically have proven bad places for criminality research because they fail to account for
the criminals who don't go to prison.
A similar thing happened in the 60's when researchers found that male inmates were more likely to have an extra Y-chromosome (XYY aneuploidy). XYY hit the media as the "criminal chromosome" and even got two guys off in court because they were "genetically predisposed to violence" and "couldn't help themselves." After further study, it turned out that the more likely explanation for the prevalence among inmates is not that XYY men are more likely to be
criminals, but that XYY criminals are more likely to get
caught and
sentenced. Their lower intelligence robs them of not only the cleverness to avoid getting caught, but the money for a good lawyer, the judgement to plead out, and other things that would help them avoid jail time.
http://pubmed-central.nih.gov/articlerender.fcgi?artid=1299297
http://en.wikipedia.org/wiki/XYY
I'm sure someone is snickering about the suggestion that theists must be less intelligent, but again, this is a correlation/causation fallacy. Less intelligent people are more likely to reject science that the don't understand in favor of superstition. In the United States, superstition masquerades as religion all the time, thus there is a social pressure among low-income less-educated people to become
tacit Christians. For all you atheists out there snickering up your sleeves, may I also propose that there is a social pressure among the educated (and pseudo-educated) to be atheist. I'd say that both of these social pressures are detrimental to the individual and society. We all need to find God (or opt not to look for Him) by ourselves.
In short, I think this study of inmates professed religiosity is worthless, except as a study of logical fallacies.