Does viewing pornography increase a person's propensity for sexual violence? There are a slew of studies saying no, while another body of research says yes. So how do we know which data to believe? Over at Psychology Today, longtime science journalist Michael Castleman explains why the studies showing an ostensible link between porn and rape are full of flaws.

Castleman's latest effort comes in response to an earlier post on evidence that sexual assault rates have declined while widespread access to pornography is growing. The post "appalled one reader," writes Castleman, "whose critique included citations of studies she claimed demonstrated a strong connection between exposure to porn and sexual assault."

The studies supplied showed, respectively, that 1) a small group of imprisoned Canadian rapists tended to have been avid adult-film consumers, 2) a small group of domestic violence victims in Philadelphia tended to report that their abusers regularly watched porn, and 3) of 62 convicted rapists in Singapore, most had consumed porn within six months before committing their assaults.

It doesn't take an advanced degree to see how extrapolating these findings to the general population is in folly. These studies might show that violent men tend to watch porn, but other studies show that most men have watched porn at some point and many watch it regularly. Since most men don't go on to become violent sexual predators, it's hard to argue that the "link" between porn and predation is one of cause and effect.

Alas, this is just the kind of junk science that culture warriors regularly cite to confuse general audiences. In the realm of sex-work research, it's common for studies to include only data on incarcerated sex workers, on those with criminal records, on those in court-ordered diversion programs, or on those using certain emergency services, and then extrapolating from them to the entire population of people who sell sex. As with the studies that purport to show the effects of porn on all consumers by studying convicted criminals, this conflation leads to reports showing drastically more dire consequences of sex work than exist in more general populations.

While retrospective studies of these sorts are cheaper and lead to faster results, they also "have major—and unavoidable—flaws," notes Castleman. Besides involving only small numbers of participants who are not representative of the whole population in question, "they're prone to recall errors. And in this case, they're plagued by a key researcher error, the assumption that viewing porn is unusual, that most men don't do it, only bad guys."

The studies favored by Castelman's critic, who was trying to show a positive link between porn and rape, were all retrospective. Meanwhile, the studies Castelman cited that showed no such link were all prospective studies, in which respondents were tracked and surveyed over time. These studies "used huge natural experiments—rape rates before and after porn became easily available—in several countries with male populations in the hundreds of millions," notes Castleman. "Those enormous numbers add to the studies' credibility. So does their consistency. In every prospective trial, as the availability of porn increased, rates of sexual assault declined."

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