More surprisingly, perhaps, the reported rate of anal sex has also increased dramatically, effectively doubling since the National Health and Social Life Survey was carried out by researchers from the University of Chicago in 1988. That study suggested around 12% of American women in the 25-29 age group had experienced anal sex in the last year; that figure has now risen to 21% (and also applies to the 30-39 age group). Some 20% of American 18- to 19-year-old girls have had anal sex at least once in their lifetimes, the new study shows, rising to more than 45% among 25-29 year olds

http://www.guardian.co.uk/lifeandstyle/2010/oct/05/sex-us-american-attitudes-survey