The Chemistry of Commitment 

The reason men want sex and women want to cuddle is all about our respective brains.

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But vasopressin does play a bigger role in stimulating fatherly behavior in male prairie voles than in females. In prairie voles, and other species of monogamous mammals, males will fight to protect the family and venture out to retrieve wandering pups. In these males, oxytocin is more related to the behavior of huddling up close with mother and pups in the nest, while vasopressin is more related to the "manly" traits of protecting the nest and keeping the kids in line.

One experiment points to the same influence of vasopressin on protective behaviors in humans as in voles: It seemed to put men on aggressive alert. In a study at Bowdoin College, psychologists gave men and women a whiff of vasopressin and then showed them photos of faces with carefully neutral expressions. The men who had sniffed vasopressin frowned more and tended to see the neutral facial expressions in photos of men as unfriendly. When the women in the study looked at photos of other women, on the other hand, their facial muscles moved into friendly expressions and they rated the strange women's photos as more friendly.

This difference provides a biological explanation for the perplexing mismatch between male and female romantic strategies. In human males, the drive to be the defenders of home and family may be as innate as it is in our mammalian cousins.

Love — His and Hers

Have you ever heard a woman say that she feels love for someone deep in her womb? That could be her uterine oxytocin receptors sending out a message. Her body, primed for childbirth and coursing with estrogen, is extremely susceptible to the bonding influence of oxytocin. This is why she may begin to feel so attached, even before sex. If all it takes is a little canoodling to get the settle-down hormone flowing through her veins, then when she makes love, she feels she's in the arms of Mr. Right. Oxytocin makes her want to snuggle up for the night — and forever.

The quiet times a woman craves with her man — gazing into each others' eyes, talking about feelings — are activities that boost oxytocin and make her feel bonded. And romance for a woman tends to involve elements that, really, relate to nesting: dim lighting, flowers, and music — all of which help create the civilized equivalent of a warm, safe place to give birth.

But the man falls more under the influence of vasopressin, so his romantic needs are quite different from hers. Excitement, danger, and the impetus to protect his woman are what make him feel bonded. All the mushy stuff that she needs seems boring and even a bit of a turnoff. The movie cliché where the hero saves a woman from peril, looks into her eyes, and passionately kisses her expresses an evolutionary truth. The testosterone and vasopressin in a man's system may keep him from turning into a cuddle bug. Instead, he's more likely to play the role of protector and warrior. That charge of testosterone and vasopressin may be the reason that after sex, while she wants to cuddle, he feels the urge to jump up and fix the car — right after he takes a little nap.

Once he's mated, vasopressin readies him to respond to challenge by amping up his sympathetic nervous system. In a challenge, vasopressin overrules the effect of oxytocin, turning off trust and friendly behavior. Meanwhile, testosterone increases his energy and self-confidence, and makes him more willing to be aggressive. Also, the man's tendency to be the disciplinarian of the family may have evolved into a social norm because it's such a comfortable part of his biology.

That twist that vasopressin gives to the male brain goes a long way toward explaining how differences between mothering and fathering might have come to be. A woman, as the sole provider of milk for babies, needs to be still to let them nurse. They have a better chance of surviving if she stays close to them, even in times of danger. The best defense may be to keep quiet and hope she isn't seen. Calming oxytocin activates her parasympathetic nervous system and helps her stay put. Once a man has impregnated his mate, he's more expendable. If he dies or is injured while fighting off a predator, he's nevertheless given the rest of the family a better shot at survival.

Translate this to modern humans and you get Mommy: warm and nurturing, happy to putter around the house, glad to settle in for a long chat; and Daddy: striding out of the house to work, jumping up in the dark to investigate things that go bump in the night.

At the same time, the mated man's roaming instincts will be at least somewhat quelled as the sweet influence of oxytocin predominates over the rangy persuasions of testosterone. In fact, Peter Gray of the University of Nevada has found that testosterone levels in married men are actually lower than those of bachelors, making them more committed to the wife and kids. This held true across cultures. Gray's hypothesis is that this reflects a shift in the man's reproductive strategy as he moves from sperm-scattering to investing in his family — and the survival of his genes. He and his colleagues have yet to determine whether mated men have lower testosterone levels because they're in relationships or whether men with lower testosterone levels are more likely to settle down with a family. In either case, the lower levels of testosterone in these family men allows for a fuller flowering of the oxytocin response.

Less testosterone and more oxytocin could make for mellower sex, and the oxytocin effect is probably what makes sex in a long-term relationship so different from those intense encounters during the early days of courtship or marriage. The nucleus accumbens, the brain system that keeps us focused on winning rewards, doesn't need to go into high gear when that reward is lying in bed next to us every night. Her body and brain, bathed in the calming effects of oxytocin, over time may tend to favor comfort over passion. He, too, is calmer and less excitable, which, overall, is a healthy state for him, even if he's less of a raging bull when it comes to sex.

If we're wired for monogamy, how come so many marriages end in divorce? How come some people do just fine in polyamorous relationships? There's wide variation in individuals, even among animals. Emory's Young has found that a high percentage of supposedly monogamous male prairie voles never mate — and this is tied to a variation in the vasopressin gene. Nurture also plays a huge role in how strong the oxytocin response is.

You may not choose traditional marriage or sexual monogamy. But we all have an innate need for oxytocin-based relationships that are very different from romantic desire. You may find such a relationship not only with a mate but also with a best friend, a mentor, a parent, or your own child.

And if this sounds like a rationale for a return to gender roles, it's not. The powerful man and passive woman are stereotypes. The human ability to shape our own fate has created handywomen, hard-charging female executives, and stay-at-home dads who are as good with a spatula as a hammer. Evolution is not destiny. But if you're a woman wondering why he won't pick up his socks, or a man tired of being nagged about picking up his socks, it may help to remember that you're hearing the whispers of eons of evolution — so you should cut the other person some slack.

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