Are we true to our mates? Or do we just SAY we are?

About a third (39% of men, 27% of women) of USA TODAY readers responding to a living in the USA survey say they've had an extramarital affair.

"My guess is more of them had an affair and didn't admit it," says Fred Humphrey, past president of the American Association for Marriage and Family Therapy.

Major studies on extramarital affairs conducted in the '40s indicated that at least 50 percent of married men in the USA and 26 percent of married women had affairs by the time they hit 40. Some researchers think the number of women having affairs is rising - possibly approaching 50 percent.

Experts say extramarital affairs rarely have happy endings. Humphrey points out that about half of married couples either divorce or separate when one spouse learns of the other's affair. Others anguish over trying to salvage the relationship.

"Learning about it results in instant pain and anger," he says. "Then there'll always be a barrier, to some extent."