More than half the letters I receive from readers are about infidelity and broken trust. A woman discovers her husband is having an affair or is involved in Web pornography. Some men tell me they are worried because their spouses spend more time with "friends" at work than at home. Sometimes infidelity is about sex; other times it is about a relationship outside the marriage that is more intimate and trusting than the one at home.

The scenarios differ, but the reactions are similar. Most anyone who has been traumatized has felt shock, anger, a sense of loss, and anxiety that it might happen again. With infidelity, however, trust is severed. And not just trust in someone you thought you knew well - also trust in oneself. Most of those letters conclude with questions like, "How can I ever trust my partner again?" or "What's wrong with my judgment that I missed this?" Without trust in one's spouse, a relationship can't grow. And without trust in one's self, everything feels pretty frightening.

The American Association for Marriage and Family Therapy reports - conservatively - that nearly 25 percent of couples have experienced infidelity. Why is it so common? Evolutionary psychologists have suggested monogamy is not natural. When people spend more time at work and less at home, coworkers become close friends and then intimate friends. Sometimes the offended spouse is shocked. More often, both partners have been unhappy for a while. Sometimes it seems that the affair is a way of getting the couple out of their pattern of unhappiness. And although many couples then break their pattern by getting divorced, the majority stay together.

Marriages that continue and even thrive after infidelity tend to have these factors in common:

Commitment. Both partners must be genuinely committed to do whatever is needed to work through the problem. They need not commit to stay in the marriage over the long haul, but they must agree to sever all threatening external relationships and commit to work on this as hard as they can, as long as they need to.

Talk. Despite the discomfort, the couple must discuss the affair. And the more they talk, the better they do! Commonly the offended spouse has lots of questions about what happened and how it happened, and sometimes he or she wants to know small, intimate details. Often the offending spouse will want to move on and "put it in the past." It never works. All questions must be answered - even if they were asked before. Which brings us to:

Honesty. Answer all questions truthfully. One's instinct may be to deny or minimize some details in order to protect one's spouse. But if the dishonesty is discovered, the partner feels betrayed all over again. It's also important for the offending spouse to do some real soul-searching, to truly try to understand what happened - not simply explain the behavior, or blame the spouse, but really understand what was happening inside. Affairs often are a byproduct of unexpressed pain or anger, or low self-esteem.

Listening. It is critical for the offended partner to be able to give voice to the hurt. But this must be done without shame or blame. It can be very difficult to look inside and talk about one's most tender feelings in front of someone who has betrayed them. It's also hard for anyone to listen to the great pain he has just caused. But achieving this level of openness and listening is critical for healing. If both parties feel understood, it can lead to the next step.

Forgiveness. Forgiveness is crucial - but useless until you're ready. It's not important to forgive the betrayal. It's important to forgive the person. And how does that happen?

Awakening. When we first fall in love with someone, dopamine is released in the brain. That's why we feel so euphoric. Our critical judgment is also impaired during this period: We are less likely to see our partner's flaws.

Just as falling in love affects the brain, betrayal also affects it so that initially we are only able to see the negatives. After much work, the brain - and the relationship - find balance.

Most people report that they see themselves and their partners differently than they did before. They see the great flaws in their spouse and also in themselves. They also see the depth of their partner's devotion to the relationship.

So we awaken to a reality that previously might have seemed an impossible contradiction: We live with someone who both loves us and can hurt us. At the same time, we now understand that we know one another at a much deeper level. Love feels more solid and trustworthy.

And there's a great learning here, about one's own ability to survive, and even grow, following a trauma like this.

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