"Office spouse" relationships often start out innocently: Coworkers grab lunch, share inside jokes, commiserate.

Sometimes, though, they go too far.

Susan Healy, a clinical therapist in Pound Ridge, N.Y., has counseled many husbands and wives who've seen their relationships rocked by inappropriate work friendships.

In one case, a husband and wife started a popular events business, in which he handled the day-to-day work and she took care of the finances. He spent long hours with one colleague, a manager, planning and running the events. They'd often stay late after everyone went home to talk about the evening. The talks turned personal, with the husband sharing that he had fallen out of love with his wife.

By the time the couple separated and sought counseling, the emotional affair had progressed to the point where the husband could see himself having children with the manager -- and the wife no longer felt comfortable walking into her own business. Still, the husband couldn't bring himself to fire the manager.

"She fulfilled his need to take care of someone, and he knew firing her would put her in a bad situation," said Healy. "The wife felt indignant, that it was her business and her husband, and this woman was replacing her in both areas."

Having a best friend at the office to confide in can be a positive -- sometimes even necessary -- element of work, said Jacqueline Olds, a psychoanalyst and associate clinical professor of psychiatry at Harvard Medical School. "Work is often so stressful, we need somebody to process things with."

The problem, Olds said, is when that close friendship is with someone of the sex you're attracted to. "Two good friends who don't think they would be attracted can accidentally fall into infatuation. It happens quite often at work. People underestimate the power of infatuation and think they can handle temptation much better than they can."

Work spouse relationships subtly evolve into emotional affairs by meeting basic human needs. "It starts out with affection, which basically means you care about the person and want to see him or her succeed and you'll be there when they need you," said Willard F. Harley Jr., clinical psychologist, president of Marriage Builders and author of "His Needs, Her Needs: Building an Affair-Proof Marriage." "Next is conversation, when that conversation turns personal. Admiration is another emotional need, where you tell the person they're great at what they do. Once someone does enough of this for you, you start looking forward to being with him the next day, can't stop thinking of him -- one thing leads to another, and next thing you know you're having sex."

Knowing better isn't always a deterrent. Harley said he's counseling a bank president who has had a work affair even though he has a long history of firing people for having affairs.

"If you work with someone daily, watching each other's backs, helping each other with the problems of life, I wouldn't say a romantic relationship is inevitable, but it sure is highly probable," Harley said. "Many people never expect it to happen, and it ruins their lives."

Recent studies show workplace coupling is becoming increasingly common. It's good news for single workers looking for love and increasingly stuck at work -- but more treacherous for those in committed relationships.

A study last year by career information site Vault.com found that 28 percent of those surveyed said they had an office "husband" or "wife," while a survey of 640 white-collar workers from digital programming and advertising firm Captivate Network found that 65 percent of the employees have or have had a "work spouse."

The Captivate study also found that the lines between personal and professional can get a little blurry, with 24 percent of those surveyed saying they continue communication on weekends and weeknights. About a third of respondents said their work spouse's appearance is important to them. Fifty-nine percent confided in their work spouse about problems at home, and more than a third discussed their sex life with their work spouse.

One participant in the Vault study wrote, "My wife even calls her [my coworker] my second wife. I talk to my coworker more during the day than I do my wife in the evening ... I even caught myself saying 'Love you' to her ending a call ... a couple times ... once in front of my wife, but thank God we all found it funny."

Not all transgressions are considered funny, as seen by the personal and professional fallout in publicized workplace affairs involving Best Buy CEO Brian Dunn and even "Snow White and the Huntsman" director Rupert Sanders and actress Kristen Stewart.

Dangerous liaisons happen out of the spotlight, too. Ruth Houston, founder of InfidelityAdvice.com and author of "Is He Cheating On You?" described a woman who first became suspicious about her husband's work spouse when she was in a bathroom stall during his office party. Houston said the woman overheard co-workers saying, "That's his wife? I didn't even know he was married. I thought he was dating so-and-so."

"Having an affair does terrible damage to the person you promised to care for. In our surveys, a spouse having an affair is listed as more painful than losing a young child, losing your arm, having your house burned down and physical abuse," Harley said. "And from a career standpoint, an affair can be devastating. I know quite a few people who have lost everything."

Healy, whose work has been highlighted in the "Ladies' Home Journal" series "Can This Marriage Be Saved?," said the husband and wife entrepreneurs she counseled did end up getting back together and salvaging their business -- without the manager. But there were warning signs before the situation got to the breaking point, she said. The wife started sensing the connection between her husband and the manager when she'd come into the office and was hearing from friends that they had seen the pair together around town.

You've crossed the line, Houston said, if you're hiding any aspect of your relationship from your spouse, meeting after work for social rather than work situations, start moving your conversations from business to personal topics, and specifically, if you start complaining about your real spouse. "That's telling the person there's an open door. If this relationship supersedes your relationship with your spouse, where you're spending more time or sharing more information with this person, that's a danger sign," she said. "And if you're reluctant to share anything you do or say with your spouse, even if it seems innocent, you have to ask yourself why."

To get right, you have to "back pedal," Houston said. "Stop having lunch. Keep your office door open. If you're on business trips, meet in the conference rooms or lobby, not in hotel rooms. If you sense anything is going on, nip it in the bud."

And if you still proclaim innocence, Houston said you can test yourself with one specific question: 'If you were single, is this the type of person you would want to be with?' If the answer is yes, that's a red flag."

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