Rape is down in the US. Why?

The good news is that incidence of rape in the United States is down 85% from 1979 levels. Violent crime dropped only 59%. Why did rape decline more than overall violent crime? Could it be a statistical mirage, due to the fact that rape is under-reported? But why would it become more under-reported in recent years? One would think the opposite would be the case, with increased awareness and education on the subject (let's hope).

This guy (Anthony D'Amato) has a guess: increasingly easy access to porn has lowered the incidence of rape by providing an outlet for sexual urges that would otherwise be channelled in a criminal direction. Here's his abstract:

The incidence of rape in the United States has declined 85% in the past 25 years while access to pornography has become freely available to teenagers and adults. The Nixon and Reagan Commissions tried to show that exposure to pornographic materials produced social violence. The reverse may be true: that pornography has reduced social violence.

I read the paper and while the thesis seems plausible, I wasn't particularly impressed with the methodology though: comparing rape incidence rates between the states with the highest Internet use and lowest Internet use. Is Internet use really a good proxy variable for porn use? I know there's a lot of smut on teh Internets, but there's other stuff too, like political blogs of unusual acumen.

Glenn Reynolds seems to find the thesis pretty plausible:

Hmm. What's different since 1970? Lots of things, of course, though bared midriffs and short-shorts are back. But probably the most relevant difference is porn. In 1970, some people argued that porn caused rape. Since 1970, though, porn has exploded. In 1970 you had to work pretty hard to find porn. Now you have to work nearly as hard to avoid it.

But rape has gone down 85%. So much for the notion that pornography causes rape — or, at least, if it did have much effect in that direction, it would be hard to explain how rape rates could have declined so dramatically while porn expanded so explosively.

So while I won't go so far as to argue that porn actually prevents rape, it seems clear that the claims of some people — including a commission headed by former Attorney General Ed Meese back in the 1980s — that pornography promotes rape are, at best, overstated. I suspect, though, that anti-pornography crusaders are unlikely to heed this lesson.

I agree that while porn may not be the cause of the decrease in rape, it's pretty hard to argue that it causes rape, given its recent ubiquity and the recent percipitous decline in rape incidents.