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

Susan knew that if her husband found out about her , the would almost certainly end. She even acknowledged that her husband, who worked in law enforcement, could get violent or aggressive. So why was she unfaithful? Susan had many reasons. The strongest was that the from these men brought her the rush that she had once felt as an exotic dancer, when men's eyes were on her all night long.

Susan is not alone. Conservative statistics report that at least 65 percent of males, and close to 50 percent of females admit to committing sexual at least once in their life. But the numbers are growing and are still widely viewed as low estimates. Despite the decreased stigma and attributed to sexual , many people simply won't report that they've cheated, even in anonymous surveys. If asked their views of sexual , most of these same people will state that is wrong, immoral, and damaging to a person and a relationship.

So why do they do it? For Susan, and many others who feel their heart and palms sweat when thinking about with someone they're not married to, it helps to understand why cheating is so powerful, compelling, and intriguing. There is not just one answer. Extramarital 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 such as how a person feels about themselves and their relationship. Susan was desperately seeking the power and she got from being desired by multiple men, along with the sense of control that their desire gave her. I've also seen people who cheat because they want to be caught so they can end their . For others, is an escape hatch, a pressure relief valve, a sense of freedom, or 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 the State University of New York found that semen works as an anti-depressant. When women don't use condoms during heterosexual , their depressive symptoms go down because their bodies absorb the shockingly long list of psychoactive present in semen, several of which work as natural . In 19th century Paris, French writer Madame Germaine de Stael once wrote to her husband that she had taken a new lover to banish the feelings of and sadness that plagued her.

When a sexual relationship starts, our brains are flooded with neurochemicals that foster feelings of excitement, obsession, and impulsivity. , PEA, and are particularly influential at this stage. When these chemicals in our , we can often do nothing but think about our new lover—we doodle their name, daydream about being with them, and take every opportunity to be near them. Sex with this new lover is exceptionally powerful, as these chemicals enhance our physical reactivity. alone makes our skin far more sensitive to touch, creating the trails of pleasure that linger after our lover's touch.

With our primary mate, our spouse, these same chemicals may have faded and been replaced by that foster thinking about long-term plans and nurturing and providing for our mate and any children. 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 a spouse can often detect infidelity by changes in our energy, mood, and increased interest in sex. In my book Insatiable Wives, I speculate that this transfer of excitement explains why many nonmonogamous couples report that extramarital sex by the wife actually reignites within their .

Sex with a new lover is often described as incredible. A male with a new lover is able to have more sex, for longer, more frequently and vigorously, and he ejaculates harder with more sperm in that ejaculate. His body attempts to compete against any other men his new lover might be with and acts like the body of an alpha male, flooded with . A woman will become intensely orgasmic, her body responding sexually and physically to a new man in ways she often has not felt in years. When she is at the most fertile point of her cycle, a woman may find herself attracted to men she would normally avoid. She may be drawn to men who have an edge of and dominance who would never truly commit to or being tied down.

All of this occurs within a social context. A may have already weathered infidelity, and so a new fling may not end it. Perhaps the husband and wife have an " " or perhaps there is little likelihood of getting caught. Perhaps you and your spouse just fought and cheating is the best way you can show how angry and betrayed you feel, even if your spouse won't find out about it.

The rush and excitement of infidelity comes from many different things, for each different person. Extramarital sex may serve 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 these influences. Whether infidelity is right or wrong for any one person is for that person to decide, within the context of their life, their or or , and their relationship.

Others often judge extramarital affairs with little of the role they serve, and even less of why people would choose to pursue them. People's reasons for infidelity have little to do with their morals or —these things may affect their decisions to pursue sex outside their marriage, and may affect how they do it.

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

Abstinence-only sexual is now widely understood to have serious limitations. One of the greatest limitations is that when these children find themselves in a situation when they decide to have sex, despite their previous vow of abstinence, they are unprepared to do so safely. If you cannot understand the of the temptations behind infidelity, or the role that extramarital sex plays in your life, your body, and your , then you cannot understand those who choose to cheat—or prepare yourself when these temptations confront you.