Some surveys say yes and some say no.

In a study published last summer in The Journal of Family Psychology, for example, researchers from the University of Colorado and Texas A&M surveyed 4,884 married women, using face-to-face interviews and anonymous computer questionnaires. In the interviews, only 1 percent of women said they had been unfaithful to their husbands in the past year; on the computer questionnaire, more than 6 percent did.

At the same time, experts say that surveys appearing in sources like women's magazines may overstate the adultery rate, because they suffer from what pollsters call selection bias: the respondents select themselves and may be more likely to report infidelity.

But a handful of new studies suggest surprising changes in the marital landscape. Infidelity appears to be on the rise, particularly among older men and young couples. Notably, women appear to be closing the adultery gap: younger women appear to be cheating on their spouses nearly as often as men.

"If you just ask whether infidelity is going up, you don't see really impressive changes," said David C. Atkins, research associate professor at the University of Washington Center for the Study of Health and Risk Behaviors. "But if you magnify the picture and you start looking at specific gender and age cohorts, we do start to see some pretty significant changes."

The most consistent data on infidelity come from the General Social Survey, sponsored by the National Science Foundation and based at the University of Chicago, which has used a national representative sample to track the opinions and social behaviors of Americans since 1972. The survey data show that in any given year, about 10 percent of married people -- 12 percent of men and 7 percent of women -- say they have had sex outside their marriage.

But detailed analysis of the data from 1991 to 2006, to be presented next month by Dr. Atkins at the Association for Behavioral and Cognitive Therapies conference in Orlando, show some surprising shifts. University of Washington researchers have found that the lifetime rate of infidelity for men over 60 increased to 28 percent in 2006, up from 20 percent in 1991. For women over 60, the increase is more striking: to 15 percent, up from 5 percent in 1991.

The researchers also see big changes in relatively new marriages. About 20 percent of men and 15 percent of women under 35 say they have ever been unfaithful, up from about 15 and 12 percent respectively.

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