Jim was angry, upset, annoyed, and then angrier still. But most of all, he was perplexed. "I don't understand it," he said. "Things were going so well." The 32-year-old businessman's discovery that his wife of three years was having an affair had sent his normal confidence soaring out the window. How could she do such a thing? And how could he not have known?

The answer, experts in the field of female infidelity say, is that he probably should have known. While cheating is about the worst thing that can happen in a relationship, it seldom occurs without warning. "Most affairs aren't sought, they `happen,' usually during a vulnerable time in the marriage," says New York state psychologist Don-David Lusterman, Ph.D., author of Infidelity: A Survival Guide. "If a man is conscious of what's going on in the relationship, it's a lot less likely."

It's hard to say how frequently married women cheat. Polls have had widely divergent results, showing that anywhere from 10 percent to 60 percent of the married female population will have an affair at some point--although marriage counselors say the lower end is more likely. But while most cheaters are still men (but not by much, actually), women are more likely than ever to be the ones who stray. According to Anthony DeLorenzo, a New Jersey-based investigator who specializes in checking out cheating spouses, the increasing number of opportunities means that the number of women who have affairs is rapidly climbing.

"With more women than ever in the workforce and on the Internet, their percentage is going up all the time. In our investigations, we follow about 68 percent males and 32 percent females, but the ratio will probably end up being 55 to 45," DeLorenzo says.

Do you know how likely it is that the sweetheart you married will have an affair? We've taken four scenarios to our experts to find out which ones are most apt to result in cuckoldry. Take a look, and learn how to prevent what happened to Jim from happening to you.

FOUR SCENARIOS, FOUR SOLUTIONS

1 The Coasting Couple

Your marriage seems pretty normal. Sure, you aren't having sex as often as before, but nobody's complaining. She doesn't mind your spending a lot of your free time with the guys or in front of the tube, and isn't always asking you to go out with her at night the way she once did. It isn't the passionate romance that it once was, but whose marriage is? All in all, things seem to be running along just fine.

Will she cheat? Possibly. While you may be able to get along "just fine" without romance in your life, many women can't. "If a woman strays, it's usually because she's looking for emotional support she feels she's missing," Lusterman says. "A woman is most likely to cheat because she feels unrequited, unresponded to, unheard. She'll say, `He doesn't know how to talk to me, doesn't pay attention to me--I can talk to my best friend better.'"

The fact that she's stopped asking you to do things with her doesn't mean she doesn't want your company anymore, but that she's given up on you. "If we know anything about why a woman will have affairs, it's that she'll consider it when she perceives her mate is not willing to invest in the relationship, and she feels she has no other option," says Howard J. Markman, Ph.D., a professor of psychology at the University of Denver in Colorado and co-author of Fighting for Your Marriage.

Avoid the horns: "The worst thing a man can do is to take his wife for granted. If you want her to be happy, you need to bring the fun, friendship and sensuality back into your relationship," Markman says. "You should be real companions," adds Lusterman. "Talk, spend time together, and have sex when you're actually awake, not just after everything else is done and you collapse in bed."