The rate of forcible rape as reported on victimization surveys peaked in 1979 at about 2.8 per 1000 population (age 12 or older). In 2009 the rate fell to 0.5:
This is an astounding decline, but I don’t know of any reason to doubt the accuracy of the numbers. The National Crime Victimization survey is conducted twice a year, with a sample size of 134,000. Rapes reported to the police have been shrinking more slowly than survey-reported rapes, and actually rose from 1979-1990, consistent with other data showing an increasing willingness of victims to report having been raped.
I have no idea what caused the decline, which is far more dramatic, and started far earlier, than the declines in other violent crimes. But the theory that pornography causes sex crime would seem to have a hard time surviving comparison with the data.
[Cross-posted at Same Facts]