Between 1991 and 2006 people over 60 and under 35 showed increased infidelity rates.
The New York Times has a piece about infidelity up today. They story cites the familiar fact that it's hard to get good data on infidelity because in face-to-face interviews respondents are less likely to admit to infidelity, and women's magazine survey takers are self-selecting, making infidelity seem more prevalent than it really is.
According to the Times, most reliable infidelity stats come from the General Social Survey, which uses a national representative sample and has been going since 1972. Overall infidelity rates have been pretty consistent—from year to year about 7 percent of women and 12 percent of men say they have had sex outside their marriage.
But infidelity actually is up in older and younger people. Between 1991 and 2006 infidelity in men and women over 60 increased from 20 to 28% and 5 to 15%, respectively. For people under 35 the numbers in 1991 were 15% for men and 12% in women, and in 2006 had risen to 20% for men and 15% for women.
For older people the increases could be due to advancements in health care like Viagra and improved hip replacements. The Times suggests that young people are cheating more because internet porn is changing what people think is normal—this sounds dubious to us, though. Porn might be changing what sex acts people think are normal, but most porn doesn't glorify cheating.
Anyway, researchers are interested in the reasons behind rising rates of infidelity among women. They say it's possible that women and men have always had the same rates of infidelity as men but haven't admitted it until recently—the old "men are studs women are sluts" double standard may come in to play here. Plus historically it's been easier for men to cheat, since they're out and about while women are isolated at home. Nowadays women work late and email at home, so they, too, can easily cheat.
They don't really know what's behind the statistics, though, so the piece end on a positive note: married couples appear to be spending more time together and having sex about once a week—58 times a year.