He knows your birthday, your favorite food, worst fear, and deepest, darkest secret. No, it’s not your husband, the man you promised to love and cherish until the day you die. It’s your office spouse -- a phrase coined to describe the new relationship phenomenon that’s developed as Americans work longer, harder, and in closer proximity with colleagues of the opposite sex.

“An office spouse meets emotional needs, going beyond the requirements of the job,” says Willard F. Harley Jr., PhD, author of His Needs, Her Needs: Building an Affair-Proof Marriage. “If you are in a bind, here is a co-worker -- someone of the opposite sex -- who will care for you, who you can depend on, and who you can confide in.”

Maintaining a healthy and nonsexual relationship with an office spouse can be tricky and sometimes detrimental to your real marriage. From walking a thin line between friendship and adultery, to avoiding a workplace husband or wife altogether, to keeping it strictly platonic, experts give WebMD the rules of engagement when it comes to the office spouse.

The Office Spouse Phenomenon

While you wouldn’t dream of cheating on your loved one to whom you are legally bound, you do work closely with someone of the opposite sex all day long, Monday through Friday, in many cases upwards of 60, 70, or even 80 hours a week. You do lunch, you talk about your life and family, and you stick together through the good times and the bad, in sickness and in health. You share your thoughts, hopes, and ambitious dreams -- there’s an intimacy between you ... yet you’re not intimate.

“It has to do with proximity,” says Harley, president of Marriage Builders, a marriage counseling organization and web site. “It is easy to care about someone of the opposite sex who is working next to you for eight hours or more a day.”

The concept of an office spouse is new but not necessarily uncommon. A survey conducted by Vault.com, a media company for career information, found that 32 percent of 693 respondents from a variety of industries reported having an office husband or wife.

“The phenomenon of the office spouse is increasing,” says Mark Oldman, co-president of Vault.com, a workplace information web site. “Only recently has it been acknowledged that you can have a relationship approaching the intimacy you have with your significant other, but at a very different level.”

Office spouses speak the same language: they get “inside jokes,” understand each other's frustration with the boss and internal bureaucracy, and can pick up on work vibes, both good and bad.

“One sense we got from the survey was that there are certain things you can share with an office spouse that are more difficult to share with a real spouse, in part because of the practicality of it,” says Oldman. “Talking about a circumstance at work requires background and personal experience that a real spouse just doesn’t have.”

So on occasion, an office spouse is more in tune with your life than a real husband or wife, which is when things can get dicey.

Crossing the Line

“The question is, how do affairs start,” says Harley. “They start as friendships, as kind of a buddy. The person is drawn to you because of your honesty and openness.”

Over the course of his career, Harley explains that thousands of people have come to him who have developed this kind of relationship at the office and had it turn into an affair.

“This is one of the reasons why I caution everyone from developing an office spouse relationship,” Harley tells WebMD.

Vault's 2006 office romance survey also suggests reason to worry: 50 percent of respondents had known a married co-worker who engaged in an affair with someone else at the office.

“I’m not opposed to males and females working together,” says Harley. “But it’s a thin line between an office spouse and an affair.”

Office Spouse Rules

If you have an office spouse, staying on the right side of the line is a must, for both your marriage and your career. Here are the rules of engagement:

-- “Don’t share personal information at work, especially information about your marriage,” says Harley. “If someone else starts to share their information with you, let them know you are not interested. It’s very hard to do, but don’t let it get personal. If someone does share personal information with you, tell your spouse about it so you’re not creating your own world at work that your spouse isn’t aware of.”

-- If you do get personal, be careful how you categorize your marriage. “If you are not getting along with your husband, and there is someone at the office who does care for you, and you tell them that, then you’re off and running,” says Harley. “If you say, ‘I am crazy about my husband and we love each other so much,’ the other person is less likely to invest time or emotion into the relationship.”

--“Don’t be alone with a person of the opposite sex separate from your job,” says Harley. “For example, don’t carpool one-on-one, don’t engage in recreational activities after work, or if you have to travel for work with one person, bring your spouse. Romantic relationships develop out of recreational activities and intimate conversations -- those are the two major hooks.”

-- “Don’t drink with your office spouse,” says Jenn Berman, PhD, a psychologist in Beverly Hills, Calif., who specializes in marriages and families. “There’s a strict no alcohol rule with the office spouse, because when you drink the lines get blurred.”

-- “Introduce your real spouse to your office spouse,” Berman tells WebMD. “Go out to dinner with your office spouse and his or her significant other, and yours. Make your real spouse included in the relationship so it doesn’t feel exclusive.”

-- “Avoid constantly talking about your office spouse at home,” says Berman. “Your real spouse should know about your office spouse, but don’t overdo it.”

According to the Vault.com news release about its romance survey, one survey respondent said, "If I talk about my 'work husband' too much and in a somewhat too positive light, my real husband starts to get suspicious and a little jealous; mind you, nothing is going on, we probably have ‘mini-crushes’ on each other, but no more."

The Reality of an Office Spouse

While an office spouse might be a great friend, a strong support system, and a shoulder to cry on, it can be risky. So ask yourself, is an office spouse worth it?

“If your spouse has an affair, it will be the worst experience of your life,” says Harley. “You want to take extraordinary precautions to make sure it doesn’t happen to you, because nothing will compare to it. So look at the conditions that make an affair possible: most affairs take place on the job, and among really close friends at work.”

It’s a risk-benefit scenario, so if your marriage is important to you, you might want to give serious consideration to keeping your office relationships professional.

“The idea of an office spouse is a huge danger -- you would not want your husband to have an office spouse,” says Harley.

While an office spouse poses a threat, it’s not beyond the realm of possibility that the relationship can remain platonic.

“We are considering a survey about whether or not these relationships turn into affairs,” says Vault.com's Oldman. “It’s a phenomenon to look at because it makes sense that an affair would be the next step.

Still, that’s not always the case, and a healthy office spouse situation is between people who would never let it get there -- who understand the line and stay far from it.”