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HELEN FISHER: Monogamy is natural. Adultery is natural too. Neither are part of the supernatural. But I don't think people really understand monogamy. Mono means one, and gamy means spouse, one spouse. Polygyny, poly means many, gyny means women, many women. We are an animal that forms pair bonds. We are basically mono-gamous, monogamous. We're also adulterous, I think we've evolved what I call a dual human reproductive strategy, and we tend to be an animal that, a creature that forms a pair bond for a period of time, breaks that pair bond and forms a new pair bond. Serial monogamy and clandestine adultery. I think we've evolved these three distinctly different brain systems for mating and reproduction: sex drive, feelings of intense romantic love, and feelings of deep attachment. They're often connected to each other. You can fall in love with somebody, drives up the dopamine system, triggers the testosterone system and all of a sudden they're the sexiest person in the whole world. But they're not always well-connected, you can lie in bed at night and feel deep attachment for one person, and then swing wildly into feelings of intense romantic love for somebody else, and then swing wildly into feeling the sex drive for somebody who you've barely ever met. Which made me wonder whether millions of years ago there was something adaptive about having a partnership with one person and raising your babies and having extra relationships with other people. And it's actually relatively easy to explain—dial back a billion years, you got a man who has got a wife, a partnership and two children, and he occasionally goes over the hill and sleeps with another woman and has two children, extra children with her. He's doubled the amount of DNA he has spread into the next generation. Those children will live and pass on whatever it is in him, some of the genetics, some of the brain circuitry to be predisposed to adultery. But why would a woman be adulterous? A lot of people think that they're not as adulterous, but every time there's a man sleeping around, he's generally sleeping around with a woman, so you got to explain women too. What would a woman have gotten if she's had a partner a million years ago and two children, she slips over the hill and has sex with another man. Well, she'll get extra goods and resources, extra meat, extra protection. If her husband gets injured and dies, one of these extra lovers might come in and help her with her children, even think some of those children are his. It's an insurance policy, and she may even have an extra child and create more genetic variety in her lineage. So the bottom line is for millions of years, there were some reproductive payoffs, not only to forming a pair bond, but also to adultery, leaving each one of us with a tremendous drive to fall in love and pair up, but also some susceptibility to cheating on the side.

CHRISTOPHER RYAN: We are designed by evolution to be titillated by erotic novelty, males and females. Given that evolutionary design, it's completely predictable that 10 years of the same thing, whether it's the same music or the same food or the same sex partner, is going to lead to resentment, discomfort, whatever. It's going to lead to a diminishment of passion, certainly. So we start with that, and then we add to that, the notion that we're taught that that shouldn't happen, that if it does happen, there's something wrong with you or something wrong with your relationship. And so people aren't expecting that to happen. And so they interpret that diminishment of passion as a failure. It's not your fault, it's not your partner's fault, it's the fault of the clash between the sort of animal we are and the sort of society we've designed. And as long as there's that conflict between our biology and our societies, there are going to be these problems.

NOEL BIDERMAN: If you look at married couples, top of their list is certainly not sex, it's things like raising their children and having a great economic situation, a home, travel, vacations, those things, your extended family. Sex is important, most people didn't sign up for a life of celibacy, they're not priests and nuns. And so it's important, but because it's not primary, but yet marriage is almost defined by this monogamous notion where it is primary, we have this massive disconnect. And so what ends up happening, is people love those first two, three, four, five factors of their lives, they do cherish everybody within it, it's just what does or doesn't go into the bedroom. that's bothering them. And so through an affair, through liaison, and even to some degree through the use of pornography and other things, they can find these cathartic outlets that allow them to stay within that marriage.

RYAN: The point of marriage is that you wanna get old with someone, you wanna share your life with someone, maybe you wanna raise children with someone, you wanna have a certain stability and trust that you couldn't possibly get with short term relationships. That's the point of marriage. And by imposing this expectation of sexual exclusivity for 40, 50, 60 years, we're cutting ourselves off from those really important things for something that's essentially trivial. Sex isn't really that important, it's not that big a deal. And by making it such a big deal, we sabotage things that really are important. So a harm reduction approach might make a lot more sense than the sort of absolutest approach that a lot of people take where any infidelity, any, you know, my husband looks at porn, that means he doesn't love me anymore, I mean, these sorts of responses to very natural behaviors cause a lot more problems than they solve, I think. I think if marriage is going to survive as an institution, it's going to certainly have to continue adapting to the realities of human nature as opposed to trying to shoehorn human nature into some predetermined shape.

ESTHER PEREL: Why do people do this? Why do people who often have been faithful for decades, one day cross the line they never thought they would cross? What's at stake? One of the great discoveries and surprises in my research for the state of affairs, was to notice that people would come and say, "I love my partner, I'm having an affair." That sometimes people even in satisfying relationships also stray, and they don't stray because they are rejecting their relationship, or because they are reacting to their relationship, but they often stray, not because they wanna find another person, but because they want to reconnect with a different version of themselves. It isn't so much that they want to leave the person that they are with as much as sometimes they want to leave the person that they have themselves become. The more parts of yourself you can bring into a relationship, the less likely you may then be to go looking for the lost parts elsewhere. And that's when I began to say even people in happy relationships cheat as well. It isn't always about the other or about the relationship. At the heart of affairs, you will find betrayal and lying and deception and loss, but you will also find yearning and longing and self discovery and it's an exploration. And it is those two experiences that make this most complex conundrum of infidelity. What it did to you and what it meant for me.