"Men are still more likely to cheat than women," said Yanyi Djamba, director of the AUM Center for Demographic Research. "But the gender gap is closing."
Blacks, executives and managers, and Southerners were most likely to report extramarital affairs to the 40-year-old survey, the oldest continuous source of data on American behavior.
The main impetus behind extramarital affairs was predictable, Djamba said: One in four men described their marriages as "not very happy," more than twice the number of wives who rationalized their adultery that way.
The survey results lend support to one researcher's argument that what's been presumed about female sexuality for centuries may be wrong. Daniel Bergner, the author of the newly published book "What Do Women Want?," said cultural expectations have prevented women from having more affairs.
"Women are programmed to seek out one good man, and men never have been really well-suited to monogamy, right?" Bergner said in a telephone interview. An increasing body of science suggests that women's sex drives are as powerful as men's libidos, Bergner said, though they've been repressed by thousands of years of male-dominated culture.
Alton Abramowitz, president of the Chicago-based American Academy of Matrimonial Lawyers, said he's seen an increase in the number of divorce cases sparked by cheating wives.
"We always had a few cases with women, but they were much more discreet about it," he said. "In the past 10 years or so, though, there's been an uptick in those cases coming through our office."
More women may feel free to cheat because the economic consequences aren't as dire as they were when more women stayed at home, said Pepper Schwartz, a University of Washington sociologist who writes "The Naked Truth" column for AARP, the largest group representing the elderly in the U.S.
"They can afford the potential consequences of an affair, with higher incomes and more job prospects," she said in an email. "They have more economic independence and may meet a better class of mate."
The ease of online affairs and the prevalence of computer use among younger women may be responsible for a large share of the increase, Schwartz said.
"Think Ashley Madison," she wrote, referring to the online affair-matchmaking service.
The website has grown since its 2002 creation to serve 3.5 million active users speaking nine languages in 26 countries, said Noel Biderman, the chief executive officer of Toronto-based Avid Life Media Inc., which operates Ashley Madison.
"There's been a cultural shift," Biderman said, "and female infidelity is very linked to cultural change."
The website's usage patterns by age highlight the shifts, he said. The ratio of males to females is greatest among users older than 65, with 14 men for every woman. The ratio is 4-to-1 among users in their 50s, 3-to-1 for spouses in their 40s, and evenly divided among people using Ashley Madison in their 30s.
The number of female affairs still lags behind male dalliances. For every two women like actress Meg Ryan, who exchanged cheating accusations with her ex-husband Dennis Quaid, or Paula Broadwell, the biographer-turned-mistress of former CIA Director David Petraeus, there are three men like former President Bill Clinton, pro golfer Tiger Woods or onetime South Carolina governor, and now U.S. congressman, Mark Sanford, who have been the focus of much-publicized reports of extramarital affairs.
As the nation's median age increases, changes in attitudes about women engaging in sex with someone not their spouse may cause the gap to narrow more, Bergner said.
"Once you strip away the stigma from the equation, interest in casual sex is about equal for women and men," he said. "So we men may have a lot to worry about."