A woman came to my office a few times over the past few months. Her husband had encouraged her to enter therapy because of his concern about how much she was drinking. But it only took a session or two to find out that there were more significant issues at play. Susan was 41, slender and professional, and her good lucks from her days as an exotic dancer still lingered. She'd been married for five years, and for the past three, each business trip she took involved her hopping into bed with at least one man. On the most recent trip, a week-long excursion to Chicago, she slept with three different male coworkers, as well as a man she met in the hotel bar.

Why? Susan knew that if her husband found out about her infidelities, divorce was likely, and the marriage would almost certainly end. She even acknowledged that her husband, who worked in law enforcement, could get violent or aggressive. So why did she do it? Susan had many reasons for her chronic extramarital sex. The strongest reason she described was that the attention from these men brought her the rush that she had felt as a nude dancer, when men's eyes were on her all night long, their attention riveted by her sexuality and confidence.

Susan is not alone. The most conservative statistics say that at least 65% of males, and close to 50% of females admit to sexual infidelity at least once in their life. These numbers are growing, and are still widely viewed as underestimates - despite the decreased stigma and fear attributed to sexual infidelity, many people simply won't report that they've cheated, even in anonymous surveys. If asked about their views of sexual infidelity, most of these same people will describe that is wrong, immoral and damaging to a person and a relationship. So why do these people do it?

I don't buy the idea that cheating, or sexual behavior of any kind, is addictive. It's too easy an answer, and it's really just a non-answer. Labeling something as an addiction doesn't help us understand it. But for Susan, and for the many other people out there who feel their hearts race and palms sweat with the idea or memory of sex with someone that they're not married to, it helps to understand why cheating is so powerful, so compelling, and so intriguing.

There is not just one answer, but many to explain why infidelity is so compelling, so powerful, so tempting. Extramarital sex involves a complex pattern of behaviors that include biological influences, psychological factors, social contexts, and the influence of the evolutionary history that shaped our sexual behaviors. Within each person, the reasons and factors vary.

Psychological factors can include things like how a person feels about themselves, and their relationship. Susan was desperately seeking the sense of power and confidence she got from being desired by multiple men, along with the sense of control that their desire gave to her. I've also seen people cheat because they want to get caught, so that their marriage can end. For others, infidelity is an escape hatch, a back door, a pressure relief valve, a sense of freedom, or sometimes, just something that "is for me, me alone. Not for my husband, my kids, my job. Just for me."

Psychological and biological reasons often interact. Researchers at SUNY have shown that semen works as an anti-depressant. When women don't use condoms during heterosexual sex, their depressive symptoms go down, as their bodies absorb the startlingly long list of psychoactive hormones present in semen. Several of these hormones work as natural anti-depressants. In 19th century Paris, French writer Madame Germaine de Stael once wrote to her husband that she had taken a new lover, as a way to banish the feelings of depression and sadness that plagued her.

When a sexual relationship first starts, our brains are flooded with neurochemicals that foster feelings of excitement, obsession and impulsivity. Oxytocin, PEA and dopamine are particularly influential at this stage. When these chemicals are raging in our brain, we can often do nothing but think about our new lover, doodling their name, daydreaming about being with them, and taking every opportunity to be near them. Sex with this new lover is exceptionally powerful, as these chemicals enhance our physical reactivity. Oxytocin alone makes our skin far more sensitive to touch, creating the sensation of trails of pleasure that linger behind our lover's touch. With our primary mate, our spouse, these same chemicals have faded, replaced by hormones that foster us to think about long-term plans, to nurture, to provide for our mate and any children that might result from the frenzy of sex that is prompted by the earlier state of our brain chemicals. A new relationship with a new lover triggers these chemicals again, and in many cases, we carry this same level of excitement and passion home with us. This is one reason spouses often detect infidelity by the changes in our energy, our moods, and even by our increased interest in sex. In my book Insatiable Wives, I speculate that this transfer of excitement explains why many nonmonogamous couples report that extramarital sexby the wife reignites sexuality within their marriage.

Sex with this new lover is often described as incredible. A male with a new lover is able to have more sex, for longer, more frequently, more vigorously and he ejaculates harder, with more sperm in that ejaculate. His body is attempting to compete against any other men his new lover might be sleeping with. His body starts acting like the body of an alpha male, flooded with testosterone. For a woman, leaving her husband at home as she takes a new lover, she is intensely orgasmic, her body responding sexually and physically to this new man in a way she often has not felt in years. Her body is responding to this new male, and to the chance of having children with him, her children who will have the benefit of the new, diverse genes he carries. When she is at the most fertile point of her cycle, the wife finds herself attracted to men she would normally avoid. Men not like her husband, who is a good caregiver, stable, calm, and nurturing to her and her children. When she's tempted to cheat, the wife is drawn to men who have an edge of aggression and dominance, men who will never truly commit to monogamy, to being tied down. And her body responds powerfully to these men, her sex becoming aroused, her heart quickening, her skin longing for his touch. She achieves orgasm with this new man within moments, and often has more orgasms with him than she may have believed possible.

All of this occurs, within a social context. The marriage may have already weathered infidelity, and this new fling isn't going to end it. Perhaps husband and wife have an "understanding." Or perhaps there is little likelihood of getting caught, and you are surrounded by friends and potential lovers, who do not see anything wrong with infidelity. Perhaps you and your spouse just fought, and cheating is the best way you can show just how angry, how betrayed you feel, even if they don't ever find out about it. Susan has a good job, and isn't financially dependent on her husband for support. If she were, it's a lot less likely she would be cheating. In Insatiable Wives, I interviewed many couples where such women, driven to cheat, had reached arrangements with their husbands, supporting the wife's extramarital adventures, creating a permissive context for her choice to sleep with other men.

The rush and excitement of infidelity comes from many different things, in many different ways, for each different person. Extramarital sex serves several psychological and biological functions, it fulfills important and powerful social roles, and we cannot understand infidelity if we do not understand it within the context of all of these various influences. Whether infidelity is right or wrong for any one person is for each person to decide, within the context of their life, their religion or spirituality or ethics, and within their relationships. But, I think that people often judge extramarital affairs with little understanding of the role they truly serve, and even less understanding of why people would choose to pursue them. People's reasons for infidelity have little to do with their morals or ethics - these things may affect their decisions to pursue sex outside their marriage, and may affect how they do it. The book The Ethical Slut is a wonderful example of this pursuit, from within an ethical, self-aware framework.

I once talked to a police officer in a methamphetamine intervention program. He said that we do children a disservice, by telling them to "Just say no," telling them that drugs are bad, that they hurt you, and damage you and your life. But we don't tell them that drugs actually create powerful positive and pleasant feelings. Then, when the kid tries drugs, they find that sense of euphoria we didn't prepare them for, and wonder what else we didn't tell them, and what else we lied about. Perhaps infidelity is the same - it feels so good, so powerful, so overwhelming, because we don't expect it. We don't understand it, because we expected it to feel sneaky, deceptive, immoral and unethical. Then, some of us feel the rush of a new lover, and crave it, despite the consequences and risks.

Similarly, abstinence-only sexual education is now widely understood to have serious limitations. One of the greatest limitations is that when these children find themselves in a situation that they decide to have sex, despite their previous vow of abstinence, they are unprepared to do so safely. If you cannot understand the nature of the temptations behind infidelity, the roles that extramarital sex plays in your life, your body, and your sexuality, you cannot understand those who choose to cheat, nor can you prepare yourself when these temptations confront you.

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