It was just before midnight one night in October when Hillary Rothrock, a 30-year-old stay-at-home mom, discovered a side of her husband she'd never known existed.

The Lancaster, Pa., couple had been to an exercise class at the YMCA, then took their two small daughters for ice cream. When they got home, Hillary Rothrock put the girls to bed, took a shower and decided to check Facebook.

"Hey, can I look at your computer for a sec?" she asked her husband, Paul Rothrock, a 30-year-old product-support representative for a social-media ad company. He was in the living room, on his laptop, and his reaction stunned her.  "No!" he hissed, pulling the computer to his chest.

Confused, she asked him again, and he became even more agitated. "You are not looking at this!" he insisted, gripping the computer tightly. That was when Hillary Rothrock realized what was wrong.

There are few moments more painful than the disclosure of an extramarital affair, an event that provokes stress and anger in both the betrayer and betrayed. What each spouse does and says in the aftermath will reverberate a long time.

It is critical to stay calm, counselors say – adding it is possible to repair a relationship after infidelity, but only if both parties are willing to work hard and honestly acknowledge shortcomings in the relationship and in themselves.

Some 20 percent of men and 14 percent of women who have ever been married have had extramarital sex, according to federally sponsored research conducted since 1972 by the social-science research organization NORC at the University of Chicago. (Reliable statistics about infidelity are scarce, largely because many people won't own up to an affair.) Paul Rothrock's affair took place by video chat and other electronic means, but it was no less sexual or emotional, he said.

How many marriages survive infidelity? Peggy Vaughan, a San Diego researcher who runs the website, surveyed 1,083 people and found 76 percent of those whose spouses had affairs were still married and living with the spouse. That figure may skew high, though: Respondents were self-selecting visitors to Ms. Vaughan's website, an "extramarital affairs resource center." Estimates from a sampling of marriage therapists range from 30 percent to 80 percent.

Several studies indicate couples in marital therapy dealing with infidelity were just as successful as couples for whom no cheating was involved, said Dr. Jay Lebow, psychologist and clinical professor at the Family Institute at Northwestern University in Evanston, Ill., who published a review of couples-therapy research in the January 2012 issue of the Journal of Marital and Family Therapy.

Whether a marriage survives an affair depends on how healthy the marriage was to begin with, how long the affair lasted and the manner in which it was discovered.

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