First, the porn:
A 10 percent increase in Net access yields about a 7.3 percent decrease in reported rapes. States that adopted the Internet quickly saw the biggest declines. And, according to Clemson professor Todd Kendall, the effects remain even after you control for all of the obvious confounding variables, such as alcohol consumption, police presence, poverty and unemployment rates, population density, and so forth.
OK, so we can at least tentatively conclude that Net access reduces rape. But that's a far cry from proving that porn access reduces rape. Maybe rape is down because the rapists are all indoors reading Slate or vandalizing Wikipedia. But professor Kendall points out that there is no similar effect of Internet access on homicide. It's hard to see how Wikipedia can deter rape without deterring other violent crimes at the same time. On the other hand, it's easy to imagine how porn might serve as a substitute for rape.(Or maybe they've just gotten hooked on World of Warcraft.)
As for violence:
What happens when a particularly violent movie is released? Answer: Violent crime rates fall. Instantly. Here again, we have a lot of natural experiments: The number of violent movie releases changes a lot from week to week. One weekend, 12 million people watch Hannibal, and another weekend, 12 million watch Wallace & Gromit: The Curse of the Were-Rabbit.
University of California professors Gordon Dahl and Stefano DellaVigna compared what happens on those weekends. The bottom line: More violence on the screen means less violence in the streets. Probably that's because violent criminals prefer violent movies, and as long as they're at the movies, they're not out causing mischief. They'd rather see Hannibal than rob you, but they'd rather rob you than sit through Wallace & Gromit.
I say that's the most probable explanation, because the biggest drop in crime (about a 2 percent drop for every million people watching violent movies) occurs between 6 p.m. and midnight—the prime moviegoing hours. And what happens when the theaters close? Answer: Crime stays down, though not by quite as much. Dahl and DellaVigna speculate that this is because two hours at the movies means two hours of drinking Coke instead of beer, with sobering effects that persist right on through till morning. Speaking of morning, after 6 a.m., crime returns to its original level.
(6 AM? Who knew criminals had this kind of work ethic?)