But the popular belief that about half the married population cheats is a myth, says a co-author of the new study, Joseph Catania, a behavioral epidemiologist.
Catania finds that about 28% of men President Clinton's age (52) have had affairs, as have about 17% of women.
One finding "popped out," Catania says. "There is very little sex with co-workers." His research shows that most affairs are with friends (57%) of at least six months, not colleagues (9%).
Only about 0.5% overall have had multiple affairs. And only about 3.3% have had extramarital sex in the past year. His sample is 55% women, 45% men; the study was done in 1996 and '97.
Catania's findings follow a 1994 report that found 80% of women and 65% to 85% of men have never had an extramarital affair. "Every study that I know of over the last few years has resulted in the same type of findings," says one of that study's co-authors, John Gagnon of the State University of New York at Stony Brook.
Atlanta psychiatrist and marital therapist Frank Pittman agrees. "Not everybody is doing it. Most men are faithful most of the time," says the author of Grow Up! (Golden Books, $23.95).
Findings on infidelity are hotly debated. Earlier studies said up to 68% of men and 66% of women confessed extramarital bed-hopping.
Gagnon says prior research wasn't based on random samples of the population, concentrating instead on people who chose to come forward to discuss their sex lives. Scientifically based random samples "tend to include more conservatives," he says.