Relationship researchers are finding evidence that may be unsettling to many married couples: Sexual affairs aren't limited to those in unhappy unions.

Among findings:

Couples who report "pretty happy" marriages are twice as likely to have had an affair as those who have "very happy" marriages, says a study in May's Journal of Marriage and Family. "Those who report 'not too happy' marriages are three times more likely than the 'very happy' to report an affair," said David Atkins, associate professor of psychology at Fuller Theological Seminary in Pasadena, Calif.

Many couples who said that either they or their spouse had an affair reported no signs of unhappiness, such as hostility, and had more positive than negative interactions, according to a study published in the June issue of the journal "Family Process" by Beth Allen, assistant professor of psychology at the University of Colorado-Denver.

Only 25 percent of men in a third study, also by Allen and reported this year in the Journal of Sex & Marital Therapy, said they had "lots of marital problems" before an affair.

So, how happy is happy enough to ward off extramarital temptation? If someone who reports being happy still cheats, is it boredom or something deeper?

Ed Diener, a psychology professor at the University of Illinois at Urbana-Champaign, said people today pursue what he calls "ultra-happiness," so they expect love to always be romantic and full of thrills.

Mira Kirshenbaum of Boston, author of a new book called "When Good People Have Affairs," says boredom in a marriage may well result in a spouse who strays. "They think their marriage is OK because they don't fight a lot, but they are distant and bored," she said. "Sex is routine. Conversation about meaningful things is like pulling teeth."

But Douglas Snyder, a psychologist at Texas A&M University, College Station, isn't convinced that boredom is to blame. He said it's a lack of closeness and passion. "People can become distant in their marriage and interpret that as boredom, but I think it's a misnomer," he said.

Atkins analyzed 1,439 responses from the 1998 General Social Survey of adults who had ever been married. He used the categories "not very happy," "pretty happy" and "very happy" to try to measure the likelihood of infidelity.

Although the general thinking has been that unfaithful spouses are less happy in their marriage and experience marital problems before the affair, the research by Allen found that couples with good relationships aren't immune from adultery.

In her study in the June issue of the journal "Family Process," Allen examined 72 heterosexual couples before marriage and followed them for five years to see who might have an affair.

Of that group, 48 couples reported no affairs, while women were unfaithful in 11 marriages and men were unfaithful in 13.

The study found that couples who later experienced infidelity weren't hostile and showed "more positive than negative communication."

Also, the study Allen did of 115 married or formerly married couples, published this year in the Journal of Sex & Marital Therapy, found that when asked whether they had "lots of marital problems" before the affair, 48 percent of women said they had, and just 25 percent of men said so.

"What's striking for men is that a lot of them were saying they weren't having marital problems" before an affair, Allen said.

Though the experts said it's natural to sometimes be physically attracted to someone else besides your spouse, they said the danger comes with acting on those feelings.

"Partners have to work hard at protecting their marriage from boredom by being vigilant and attending to the marriage on a daily basis," said Snyder, who celebrated his 34th wedding anniversary last month.