If you cheated on your spouse, would you admit it to a researcher?

That question is one of the biggest challenges in the scientific study of marriage, and it helps explain why different studies produce different estimates of infidelity rates in the United States.

Surveys conducted in person are likely to underestimate the real rate of adultery, because people are reluctant to admit such behavior not just to their spouses but to anyone.

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.

Theories vary about why more people appear to be cheating. Among older people, a host of newer drugs and treatments are making it easier to be sexual, and in some cases unfaithful — Viagra and other remedies for erectile dysfunction, estrogen and testosterone supplements to maintain women’s sex drive and vaginal health, even advances like better hip replacements.

“They’ve got the physical health to express their sexuality into old age,” said Helen E. Fisher, research professor of anthropology at Rutgers and the author of several books on the biological and evolutionary basis of love and sex.

In younger couples, the increasing availability of pornography on the Internet, which has been shown to affect sexual attitudes and perceptions of “normal” behavior, may be playing a role in rising infidelity.

But it is the apparent change in women’s fidelity that has sparked the most interest among relationship researchers. It is not entirely clear if the historical gap between men and women is real or if women have just been more likely to lie about it.

“Is it that men are bragging about it and women are lying to everybody including themselves?” Dr. Fisher asked. “Men want to think women don’t cheat, and women want men to think they don’t cheat, and therefore the sexes have been playing a little psychological game with each other.”

Dr. Fisher notes that infidelity is common across cultures, and that in hunting and gathering societies, there is no evidence that women are any less adulterous than men. The fidelity gap may be explained more by cultural pressures than any real difference in sex drives between men and women. Men with multiple partners typically are viewed as virile, while women are considered promiscuous. And historically, women have been isolated on farms or at home with children, giving them fewer opportunities to be unfaithful.

But today, married women are more likely to spend late hours at the office and travel on business. And even for women who stay home, cellphones, e-mail and instant messaging appear to be allowing them to form more intimate relationships, marriage therapists say. Dr. Frank Pittman, an Atlanta psychiatrist who specializes in family crisis and couples therapy, says he has noticed more women talking about affairs centered on “electronic” contact.

“I see a changing landscape in which the emphasis is less on the sex than it is on the openness and intimacy and the revelation of secrets,” said Dr. Pittman, the author of “Private Lies: Infidelity and the Betrayal of Intimacy” (Norton, 1990). “Everybody talks by cellphone and the relationship evolves because you become increasingly distant from whomever you lie to, and you become increasingly close to whomever you tell the truth to.”

While infidelity rates do appear to be rising, a vast majority of people still say adultery is wrong, and most men and women do not appear to be unfaithful. Another problem with the data is that it fails to discern when respondents cheat: in a troubled time in the marriage, or at the end of a failing relationship.

“It’s certainly plausible that women might have increased their relative rate of infidelity over time,” said Edward O. Laumann, professor of sociology at the University of Chicago. “But it isn’t going to be a huge number. The real thing to talk about is where are they in terms of their relationship and the marital bond.”

The General Social Survey data also show some encouraging trends, said John P. Robinson, professor of sociology and director of the Americans’ Use of Time project at the University of Maryland. One notable shift is that couples appear to be spending slightly more time together. And married men and women also appear to have the most active sex lives, reporting sex with their spouse 58 times a year, a little more than once a week.

“We’ve looked at that as good news,” Dr. Robinson said.