A paper came out of Northwestern University Law School succintly titled "Porn Up, Rape Down." The bold 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.
As soon as I read this, I immediately thought of my political statistics class at Brown where the professor (who shadily made us buy the course packet directly from him--cash only) drilled this refrain into my feeble mind: "A correlation is not necessarily a causal factor."