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.