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Ask Dr. Darwin

Relationship advice from an evolutionary perspective By Dr. Chris Ryan

Dr. Chris Ryan

Dear Dr. Darwin: I love my husband, and we have a good sexual connection, but I can’t help thinking about sex with other men. What’s wrong with me?

Sexual monogamy is a lot like vegetarianism. Nobody’s denying that an all-veggie diet (like monogamy) can be an excellent approach to life for many reasons, ranging from ethical to environmental. But the evidence (our teeth, jaw strength, digestive system, the diets of pre-agricultural hunter-gatherers, comparisons to closely related primates, and so on) quite clearly indicates that our ancestors evolved as omnivores. That’s why some of the most committed vegetarians still crave meat sometimes.

Similarly, your love for your husband, no matter how profound and sincere, will not eliminate your innate yearning for erotic novelty. Just as our teeth and intestines indicate that our ancestors ate plenty of meat, the evolved design of our reproductive organs, our orgasmic capacities, and our species-wide penchant for frequent non-reproductive sex tell us that our ancestors were sexual omnivores with voracious appetites.

The German philosopher Schopenhauer wrote, “People can do what they want but they cannot want what they want.” Given the realities of human sexual evolution, you can choose whether or not to live monogamously, but you cannot choose whether or not it will come naturally and easily. It probably won’t.


I’m pretty sure my partner is having an affair. What should I do?

’Til death do us part may be a wonderful ideal, but it's anything but an easy (or natural) path for most human beings. Yes, we are moral beings (most of us) with the capacity to override our evolved predispositions to some extent, but maybe, just maybe, an occasional slip on that long and arduous path is to be expected and forgiven.

Or maybe not. Such notions of tolerance are actively discouraged in America. As Pamela Druckerman explains in Lust in Translation, her survey of global attitudes toward infidelity, "It has come to seem obvious to Americans that the discovery of infidelity leads to a confrontation, followed by counseling, perhaps other forms of support, and a long period of discussion and recovery (sometimes in perpetuity)." Druckerman argues that this "scripted response" to infidelity is promoted by the marriage-industrial complex. She writes, "Just as the military-industrial complex needs wars, the marriage-industrial complex needs adulterous couples to believe they require help from professionals." And she's just talking about the couples who are trying to stay together. Those who decide to throw in the towel engage the even more expensive divorce-industrial complex.

So, with both human evolution and global variations in response to infidelity in mind, let's consider a few things an affair may not mean.

1. Your marriage sucks

Maybe it does; maybe it doesn't. But let's be honest: all relationships suck sometimes. If you don't know that, you haven't been paying attention. Sartre said, "Hell is other people." Sometimes, that other person is your spouse. But that's not the only reason people have affairs. The main reason people have affairs is that they can. Or at least they think they can. It's utterly normal for all of us to yearn for a little "strange" every once in a while. It's quite possible that the affair is not a reaction to you or rejection of your marriage at all, just a response to an unexpected opportunity.

2. Your marriage is over

Look, if she slept with your best man on your wedding night, yeah, you might want to consider asking for a refund on the tux, changing your name, and moving to Tasmania. But forget about the sex for a moment (I know, easier said than done, but still). How bad was the behavior—apart from the sex? Did she (or he) humiliate you publicly? Was there a lot of complicated lying going on? Did the affair or indiscretion threaten your career? Would you have been open to a heart-to-heart conversation about the natural appetite for sexual novelty if she (or he) had had the courage to initiate it? Do your kids really need to suffer over this? Can you see any way to turn this into an opportunity to get closer to each other, to break through the accumulated dailiness of life and talk about the eternal passions that brought you together in the first place?

3. He/she doesn't love you

Most men who admit to having affairs report being happier in their marriages than men who claim they've never had an affair. Sure, they could be lying (again), but maybe they're not. Maybe, like a dog with room to run, they're happy to come home in a way a dog chained to a tree can't imagine. 

Many of the women who report having affairs talk about feeling wanted and desired in a way they just don't feel at home anymore. It's not that the other guy is better, he just yearns for her more than hubby does. Totally understandable, right? But desire isn't love and ravenous hunger doesn't last long once you start eating. Let's all keep that in mind.

4. It's your fault

No, it's not your fault that your partner feels the call of the wild occasionally. It's not an indictment of you, your partner, or your marriage. It's just a predictable consequence of the fact that you're both Homo sapiens. Nothing shameful in that. (Unless you're into Original Sin, in which case, pretty much everything is shameful.)

5. Your partner is sick

Not necessarily. Your partner is a human being—a creature with millions of years of casual, promiscuous libido flowing through his/her veins. In some cases, an affair may be abusive or insanely stupid, but in others, it may be nothing more than a momentary lapse in judgment. If the latter, maybe we should consider cutting each other some slack as a way to hold our most important relationships together rather than insisting on a zero-tolerance policy that often results in greater suffering for everyone concerned.


Hey Dr. Darwin! I recently stopped taking birth control pills, and I feel like my attraction for my husband has fallen off a cliff. What’s going on?

Every woman knows her menstrual cycle can have profound effects on her eroticism. Spanish researchers confirmed that women experience greater feelings of attractiveness and desire around ovulation, while others have reported that women find classically masculine faces more attractive around ovulation, opting for less chiseled-looking guys when not fertile. 

Since the birth control pill affects the menstrual cycle, it's not surprising that it may affect a woman's patterns of attraction as well. In 1995, Swiss biological researcher Claus Wedekind published the results of what is now known as the "Sweaty T-shirt Experiment." He asked women to sniff T-shirts men had been wearing for a few days, with no perfumes, soaps, or showers. Wedekind found, and subsequent research has confirmed, that most of the women were attracted to the scent of men whose major histocompatibility complex (MHC) differed from her own. This preference makes genetic sense in that the MHC indicates the range of immunity to various pathogens. Children born of parents with different immunities are likely to benefit from a broader, more robust immune response themselves.

The problem is that women taking birth control pills don't seem to show the same responsiveness to these male scent cues. Women who were using birth control pills chose men's shirts randomly or, even worse, showed a preference for men with similar immunity to their own.

Consider the implications. A couple meets when the woman is on the pill. They go out for a while, like each other a lot, and then decide to get together and have a family. She goes off the pill, gets pregnant, and has a baby. But her response to him changes. There's something about him she finds irritating—something she hadn't noticed before. Maybe she finds him sexually unattractive, and the distance between them grows. But her libido is fine. She gets flushed every time she gets close enough to smell her tennis coach. Her body, no longer silenced by the effects of the pill, may now be telling her that her husband (still the great guy she married) isn't a good genetic match for her. But it's too late. They blame it on the work pressure, the stress of parenthood, each other . . .

Because this couple inadvertently short-circuited an important test of biological compatibility, their children may face significant health risks ranging from reduced birth weight to impaired immune function. How many couples in this situation blame themselves for having "failed" somehow? How many families are fractured by this common, tragic, undetected sequence of events?

Dr. Chris Ryan is the host of the Tangentially Speaking podcast and the bestselling author of Sex at Dawn and Civilized to Death. Learn more at chrisryanphd.com.

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