Why Women Have Sex (15 page)

Not long ago, researchers for the first time connected oxytocin with why some animals are naturally monogamous and others are not. Only about 3 percent of nonhuman mammals form monogamous bonds; the majority mate with many different partners. Some species of prairie voles form long-lasting pair-bonds (sometimes for life). They share nests, avoid meeting other potential mates, and rear their offspring together. Closely related to the monogamous prairie voles are the montane voles, which display a very different mating style. They do not form pair-bonds, and the males are uninterested and uninvolved in parental care. Female montane voles are not exactly devoted parents either—they abandon their offspring shortly after birth.

Given that these two species of voles share 99 percent of the same genes, making them very genetically similar, why do they behave so differently? As it turns out, the voles differ greatly in how they produce and process oxytocin and vasopressin. The attachment-prone, faithful prairie voles have a lot more of these bonding hormones and have a denser supply of receptors in the brain that can detect and use them.

Very recently, it has also been discovered that in prairie voles (but not in the unfaithful montane voles), the area of the brain that is loaded with receptors for oxytocin and vasopressin is also rich in receptors for dopamine, a chemical produced in the brain that has long been associated
with reward. When animals (including humans) engage in behaviors such as eating, drinking, and sex—behaviors that are necessary for survival and reproduction—their brains release dopamine. This dopamine surge makes them feel good, essentially rewarding them for behavior that increases the chances they will want to eat, drink, and have sex again. The fact that the faithful prairie voles have these “reward” receptors in the same area of the brain as the “bonding” receptors suggests that having sex with a familiar vole mate is more rewarding than having sex with a new vole. The montane voles, which don’t have bonding receptors in the same area of the brain as their reward receptors, would not associate familiarity with feeling good.

Working with these two species of voles, researcher Miranda Lim and her colleagues at Emory University in Atlanta made an amazing discovery: They found that they could turn the normally faithful male prairie voles into regular Don Juans by simply blocking the bonding receptors in their brains. They were also able to do the reverse. When they used a harmless virus to transfer the bonding receptor gene from the prairie vole to the montane vole, the montane vole showed an increase in the number of bonding receptors in the reward area of the brain. And guess what? The normally promiscuous montane vole displayed a strong preference for his current partner over novel females and was ready to settle down and raise the offspring.

Sex stimulates the release of oxytocin and vasopressin in humans just as it does in voles, but can hormonal differences explain why some humans are monogamous by nature and others are not? The Meston Lab is working hard to answer this question. Researcher Lisa Dawn Hamilton has tested whether there are differences in the brains of monogamous and nonmonogamous people. People who were considered monogamous not only chose and preferred to have sex with their current pair-bond, they also did not fantasize or “secretly” lust over other people. Nonmonogamous people, on the other hand, had a pattern of dating multiple partners at the same time or having repeated sexual relationships outside of their primary relationship.

To conduct the study, the Meston Lab team scanned the brains of people identified as monogamous and nonmonogamous while they viewed a series of photographs that depicted a variety of scenes. The
images showed erotic scenes (e.g., couples making love), romantic/bonding scenes (e.g., couples holding hands or laughing together), and neutral scenes (e.g., a rural landscape). We then looked to see whether there were differences in activation in areas of the brain known to be rich in reward receptors. We predicted that monogamous persons would show more brain activation in the reward areas when they were shown photos depicting romantic or emotional bonding scenes than would nonmonogamous persons. We expected that sexual pictures would be rewarding to both groups of people, and that they would be more rewarding than the neutral scenes. So far, we have completed the study only in men, but the Meston Lab’s predictions have been supported. The reward areas in monogamous men’s brains lit up like Christmas trees in response to both the sexual photos and the emotional bonding photos. In sharp contrast, the nonmonogamous men’s brains lit up only to the sexual stimuli; they showed very little activation in the reward areas of the brain to the emotional bonding photos. The Meston Lab needs to test this finding in a lot more men before we can conclude that there are “prairie” men and “montane” men, but it does seem that emotional bonding is more rewarding at a very basic biological level for some men than for others.

While the sexual motivations of voles and men seem to have a lot in common, human sexuality is also shaped by culture, from the way people feel about sex to their ideas about emotional connection. That’s especially true when it comes to religion.

The role of sexuality varies widely across religious denominations, with some traditions being much more sexually restrictive than others. The Book of Leviticus, which is part of the Jewish Torah and the Christian Old Testament, has been fundamental in shaping how religion and sex are linked in the minds of Americans. According to Leviticus, God gave Moses a list of prohibited sexual behaviors, along with appropriate punishments for transgressions, which often involved death by stoning or burning. The forbidden behaviors included adultery, incest, sex during menstruation, sex between men, and sex with animals. Leviticus did not prohibit marital sex. In fact, numerous passages in the Old Testament
attest to the positive moral status of the marriage bond and of sex within that bond. Leviticus also did not prohibit sex between unmarried men and women. Other biblical passages, however, made it clear that women who were not virgins when they married could be executed (Deuteronomy 22:13–29). No equivalent punishment was laid down for men who were not virgins at marriage.

It is well documented in the field of psychology that violating strict religious guidelines can lead to sexual guilt, which can impair a woman’s ability to enjoy sex. Thus it was refreshing to hear from women in our study that the association between sex and religion can be an intensely positive experience. For some women, feeling connected to their partners during sex also made them feel connected to God:

In Jewish law, it is a mitzvah [good deed] to have sex with a partner on Shabbat, and in Jewish mysticism, there is a form of sexual ecstasy that mimics the union of God and man, and recreation of the world. I can’t really describe this experience. . . . But pure joy and connection with another person I feel is becoming closer to the cycles of life and the underlying palpable energy of the world . . . in essence, God.

—predominantly heterosexual woman, age 21

It was a dream come true, being with this incredible man. I was able to lose myself and see God, where the edges of the dreamworld and the real world met.

—heterosexual woman, age 23

I had been thinking about how if God is immanent that meant Christ was in me, and in everyone. If Christ was in me, then he would also be in my partner. I suddenly had this moment where I realized that if we joined ourselves it could be Christ seeking Christ and how beautiful that would be.

—predominantly heterosexual woman, age 20

For other women, sex did not meet their spiritual expectations:

I grew up in an environment where we didn’t talk about God—or sex. For that reason I lumped them into the same category of things
that seemed special because they were beyond me. When I started dating my boyfriend, I made him wait for a very long time. When it finally happened, I expected the experience to have an almost religious quality to it. It didn’t.

—heterosexual woman, age 21

Some insight on how sex is able to create a feeling of connection with God might be found in what happens in the brain during deep religious experiences. In their book
Why God Won’t Go Away
, radiology professor Andrew Newberg and psychiatrist Eugene D’Aquili observed that in people who are seeking a deep spiritual connection through prayer or meditation—such as Franciscan nuns and Buddhist monks—an area of the brain called the parietal lobe quiets. The parietal lobe is responsible for collating sensory information—helping us to understand how visual information lines up with spatial environment, for instance. A less active parietal lobe decreases the body’s ability to orient itself in physical space and, according to Newberg and D’Aquili, to distinguish between self and nonself. Perhaps some people are so bombarded with visual and spatial information during sex that it creates a somewhat similar experience.

On the other hand, artistic depictions of religious experience, such as the Baroque sculpture
The Ecstasy of St. Theresa
by Gianlorenzo Bernini, also hint at a centuries-old association between religious ecstasy and orgasm—even if enjoying sex for its own sake wasn’t part of the equation for most religiously observant women of the day.

Although in our initial study we did not find substantial differences between men and women in the
frequency
with which they had sex for emotional bonding reasons, a study conducted in the Buss Lab showed a big gender difference in the
importance
placed on emotional connectedness with a sexual partner. In the study, heterosexual men and women from many different countries were asked a provocative question:

Please think of a serious or committed romantic relationship that you have had in the past, that you currently have, or that you
would like to have. Imagine that you discover that the person with whom you’ve been seriously involved became interested in someone else. What would upset or distress you more: a) imagining your partner forming a deep emotional attachment to that person or b) imagining your partner enjoying passionate sexual intercourse with that other person?

Hands down, women were more distressed by thinking about their partners being emotionally attached to someone else than by thinking about their partner having sex with someone else. This makes perfect evolutionary sense. From a woman’s viewpoint, a man having sex with another woman may or may not mean that he is emotionally attached to her—it could simply involve physical gratification. But if a man is emotionally attached to another woman, there is a good chance he is (or will soon be) also having sex with her. If a man is both emotionally attached to
and
having sex with another woman, there is a high probability that he will begin to reallocate his commitment and resources to her instead of to his current partner—a clear threat, in evolutionary terms.

It is worth pausing to recall that, despite our earlier example of prairie voles, sex for the vast majority of species on this planet involves no commitment whatsoever. Humans are the rare exceptions, even among primates, in being one of the few species in which males and females form long-term pair-bonds that last years, decades, and sometimes a lifetime. Among chimpanzees, the primates that are genetically closest to humans, sex occurs primarily when a female enters estrus. During this period of ovulation, the female chimpanzee’s bright red genital swelling and scents send males into a sexual frenzy, but outside of estrus, male chimps are largely indifferent to females. Thus, sexual relationships among chimpanzees are short-lived.

Among humans, ovulation is concealed or cryptic, at least for the most part. Although there may be subtle physical changes in women—a slight glowing of the skin or an increase in women’s sexual desire—there is little scientific evidence that men can reliably detect when women are ovulating. From an evolutionary standpoint, successful ancestral men typically would have needed to stick around to have sex throughout a woman’s menstrual cycle. Without cues to ovulation, a single act of sex
results in conception only 3 to 4 percent of the time. Stopping by for an afternoon romp rarely paid reproductive dividends. For this reason, some researchers believe that concealed ovulation probably evolved as a way to increase pair-bonding or commitment in human sexual relationships. This in turn increased the chances that resources would be allocated to a single mate and her children.

But that doesn’t explain the powerful emotion of love and why it evolved in humans. Evolutionary psychologists believe that it may be a form of “long-term commitment insurance.” If your partner were blinded by an uncontrollable emotion that could not be helped or chosen, an emotion that was elicited only by you and no other, and one that was made all the more powerful by its association with a cascade of sex-triggered hormones, then commitment would be less likely to waver in sickness as well as in health and if poorer rather than richer. On the other hand, if a partner chooses you based on mostly “rational” criteria—say, your access to resources or your lack of resource-eating offspring—he or she might leave you on the same basis—in favor of a competitor with slightly more desirable qualities.