Pornography and rape
One argument sometimes offered for laws against pornography, or against some kinds of pornography, is the claim that consumption of pornography leads to violence against women. A counter argument is that it has the opposite effect, that imaginary sex, including imaginary violent sex, is a substitute for the real thing.
I recently attended a talk by an academic who had found an ingenious way of using a natural experiment to find out which was true. Access to the internet makes pornography more readily available—not only cheaper and easier to find, but more private and so less likely to lead to embarassment and other negative social consequences. Internet access increased at different rates in different states. Data on rape rates by state is available. So he looked to see whether, controlling for other variables, increased access to the net correlated with an increase or decrease in rape. It correlated with a decrease—about a ten percent drop in rape for a ten percentage point increase in internet access, with the exact numbers varying according to just how he did the analysis. There was no similar relation for murder, which suggests that the result is not simply picking up the effect of some third variable that correlates with both internet access and violent crime.
As further evidence, the author repeated the analysis, separating out the data according to the age of the perpetrator. His conjecture was that the men most benefitted by the availability of internet porn would be young adult men living with their parents, since they would find it difficult to consume other forms of pornography without risking discovery. The results supported the argument; the reduction in rape was concentrated in the 15-19 age group.
Readers interested in the details can follow the link at the beginning of this post and read the original paper.