The Female Brain (12 page)

Mothers often warn their daughters not to get too close too soon with a new boyfriend, and this advice may be wiser than they realize. The act of hugging or cuddling releases oxytocin in the brain, especially in females, and likely produces a tendency to trust the hugger. It also increases the likelihood that you will believe everything and anything he tells you. Injecting the hormone oxytocin or dopamine into the brain of a social mammal can even induce cuddling and pair bonding behavior without the usual prerequisite romantic love and sexual behavior, especially in females. And consider a Swiss experiment in which researchers gave a nasal spray containing oxytocin to one group of “investors” and compared them with another group who got a placebo nasal spray. The investors who got oxytocin offered up twice as much money as did the group who got only the placebo. The oxytocin group was more willing to trust a stranger posing as a financial adviser—feeling more secure that their investment would pay off. This study concluded that oxytocin triggers the trust circuits in the brain.

From an experiment on hugging, we also know that oxytocin is naturally released in the brain after a twenty-second hug from a partner—sealing the bond between the huggers and triggering the brain’s trust circuits. So don’t let a guy hug you unless you plan to trust him. Touching, gazing, positive emotional interaction, kissing, and sexual orgasm also release oxytocin in the female brain. Such contact may just help flip the switch on the brain’s romantic love circuits. Estrogen and progesterone dial up these bonding effects in the female brain, too, by increasing oxytocin and dopamine. One study has shown that on different weeks of the menstrual cycle females get more of a rewarding jolt out of their brain chemicals. These hormones then activate the brain circuits for loving, nurturing behavior while switching off the caution and aversion circuits. In other words, if high levels of oxytocin and dopamine are circulating, your judgment is toast. These hormones shut the skeptical mind down.

The drive to fall in love is always hovering in the background. Being in love, however, requires making room in your life and your brain for the beloved, actually incorporating him into your self-image via the brain’s attachment and emotional memory circuits. As that process unfolds, less oxytocin and dopamine stimulation is needed to sustain the emotional bond. So spending twenty-four hours a day locked in an embrace is no longer necessary.

The basic drive for romantic attachment is hardwired in the brain. Brain development in utero, the amount of nurturing one receives in infancy, and emotional experiences all determine variations in the brain circuits for loving and trusting others. Melissa knew that her father was a philanderer, and that made her even more skeptical about falling in love and becoming attached. An individual’s readiness to fall in love and then form an emotional attachment can thus be affected by the brain circuit variations caused by experience and the hormonal state of the brain. Stress in the environment can help or hinder forming an attachment. The emotional attachments and bonds we make to our early nurturing figures last a lifetime. Those early nurturing figures become part of our brain circuits via the reinforcement provided by repetitive physical and emotional caretaking experiences or their lack. Safety circuits are formed based on these experiences with nurturing, predictable, secure figures. Without those experiences, there is little or no safety circuit formation in the brain. One could still fall in love for the short term, but long-term emotional attachment may be harder to achieve and sustain.

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How does the pressing reality of the “I’ve gotta have him every minute of the day” feeling in the brain transmute to an “Oh, hi, you again, sweetie. How’s everything?” state of mind? The hormone rushes of dopamine in the brain gradually calm down. If we had an MRI scanner to view the brain changes that occur when a woman goes from a state of early romantic love to a state of long-term coupling, we’d see the reward-pleasure circuits and the throbbing hunger-craving circuits dim down, while the attachment and bonding circuits would light up to a warm yellow glow.

We know the rapturous feelings of passionate love don’t last forever—and for some, the loss of intensity can be disarming. This is how I met Melissa. After she had been involved with Rob for a year, she came to see me. She explained that, for the first five months, she and Rob had had wonderful, exciting sex every day and looked forward to each minute they spent together. Now they were living together, working at demanding jobs, and starting to talk about marriage and a family. But she had begun to “feel flat” about the relationship. Her gut feelings weren’t giving her that certainty anymore. It was alarming to her that she didn’t have as much interest in sex. Not that she had found or even wanted someone else. It was just that now, compared especially with the first five months of their relationship, things lacked the passion and excitement she had grown to expect. What was “wrong” with her? Was Rob the right guy? Was she normal? Could she ever be happy with him long-term if the sexual spark and intense gut feelings in their relationship were gone?

Many people, like Melissa, think the loss of the romantic high of early love is a sign that a couple’s relationship is going south. In reality, however, the pair may be just moving into an, important, longer-term phase of the relationship, driven by additional neurological circuits. Scientists argue that the “attachment network” is a separate brain system—one that replaces the giddy intensity of romance with a more lasting sense of peace, calm, and connection. Now in addition to the exciting pleasure chemicals of the reward system, such as dopamine, the attachment and pair-bonding system regularly triggers the release of more of the bonding chemical oxytocin, keeping partners seeking the pleasure of each other’s company. Those brain circuits for long-term commitment and bond maintenance become more active. When researchers at University College, London, scanned the brains of people who were in love relationships for an average of 2.3 years, they found that, rather than the dopamine-producing brain circuits of passionate love, other brain areas, such as those linked to critical judgment, lit up. Activity in the brain’s attachment circuit is maintained and reinforced over the ensuing months and years by mutually pleasurable and positive experiences, all of which release oxytocin.

From a practical perspective, this shift from head-over-heels love to peaceful pair bonding makes sense. Caring for children, after all, would be close to impossible if mates continued to focus exclusively on each other. The downshift in love’s mania and sexual intensity seems tailor-made to promote our genes’ survival. It’s not a sign of love grown cold, it’s a sign of love moving into a new, more sustainable phase for the longer term, with bonds forged by two neurohormones, vasopressin and oxytocin.

Social attachment behavior is controlled by these neurohormones, made in the pituitary and the hypothalamus. The male brain uses vasopressin mostly for social bonding and parenting, whereas the female brain uses primarily oxytocin and estrogen. Men have many more receptors for vasopressin, while women have considerably more for oxytocin. To bond successfully with a romantic partner, males are thought to need both these neurohormones. Stimulated by testosterone and triggered by sexual orgasm, vasopressin boosts a male’s energy, attention, and aggression. When men in love experience the effects of vasopressin, they have a laserlike focus on their beloved and actively track her in their minds’ eyes, even when she isn’t present.

Women, by contrast, are able to bond with a romantic partner once they experience the release of dopamine and oxytocin, triggered by touching and the giving and receiving of sexual pleasure. Perhaps keeping my feet warm isn’t my husband’s
primary
responsibility in bed, but cuddling to release oxytocin is. Over time, even the sight of a lover can cue a woman to release oxytocin.

The exceptional bonding power of oxytocin and vasopressin has been studied in great detail by Sue Carter in those furry little mammals called prairie voles, who form lifelong mating partnerships. Like humans, the voles are filled with physical passion when they first meet and spend two days indulging in virtually nonstop sex. But unlike in humans, the chemical changes in the voles’ brains can be examined directly in the course of this frolicking. These studies show that sexual coupling releases large amounts of oxytocin in the female’s brain and vasopressin in the male’s. These two neurohormones in turn increase levels of dopamine—the pleasure chemical—which makes the voles love-struck only for each other. Thanks to that strong neurochemical glue, the pair is mated for life.

In both males and females, oxytocin causes relaxation, fearlessness, bonding, and contentment with each other. And to maintain its effects long-term, the brain’s attachment system needs repeated, almost daily activation through oxytocin stimulated by closeness and touch. Males need to be touched two to three times more frequently than females to maintain the same level of oxytocin, according to a study by the Swedish researcher Kerstin Uvnäs-Moberg. Without frequent touch—for example, when mates are apart—the brain’s dopamine and oxytocin circuits and receptors can feel starved. Couples may not realize how much they depend on each other’s physical presence until they are separated for a while; the oxytocin in their brains
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them coming back to each other, again and again, for pleasure, comfort, and calm. No wonder Rob flew off to L.A.

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Vole studies have also highlighted attachment differences between males and females. For female prairie voles, pair bonding works best under conditions of low stress. For males, high stress works better. Researchers at the University of Maryland found that if a female prairie vole is put through a stressful situation, she won’t bond with a male after she mates with him. If a male prairie vole is stressed, however, he’ll quickly pair up with the first available female he finds.

In humans, too, male love circuits get an extra kick when stress levels are high. After an intense physical challenge, for instance, males will bond quickly and sexually with the first willing female they lay eyes on. This may be why military men under the stress of war often bring home brides. Women, by contrast, will rebuff advances or expressions of affection and desire when under stress. The reason may be that the stress hormone cortisol blocks oxytocin’s action in the female brain, abruptly shutting off a woman’s desire for sex and physical touch. For her, nine months of pregnancy followed by caring for an infant under stressful conditions makes less sense than the quick deposit of sperm does for him.

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The love lives of different subspecies of voles also offer insights into brain mechanisms for monogamy, a trait that’s shared by only 5 percent of mammals. Prairie voles are champion couplers, forming monogamous, lifelong pair bonds after their marathon copulations. Montane voles, by contrast, never settle down with a single partner. The difference, scientists have discovered, is that prairie voles have what amounts to a gene for monogamy, a tiny piece of DNA that montane voles lack. As her relationship with Rob became more serious, Melissa began to worry. Was Rob a prairie vole or a montane vole?

As far as researchers know, human males represent behaviors on a spectrum from totally polygamist to totally monogamous. Scientists speculate that different genes and hormones may account for this variability. There is is a gene that codes for a particular type of vasopressin receptor in the brain. Prairie voles that carry this gene have more of the receptors in their brains than do montane voles; as a result, they’re much more sensitive to the pair-bonding effects of vasopressin. When researchers injected this “missing” gene into the brains of montane voles, the normally promiscuous males instantly turned into monogamous, pair-bonded, stay-at-home dads.

Males who had a longer version of the vasopressin receptor gene showed greater monogamy and spent more time grooming and licking their pups. They also showed greater preference for their partners—even when given the chance to run off with a young, fertile, and flirtatious female. Males with the longest gene variation are the most reliable and trustworthy partners and fathers. The human gene comes in at least seventeen lengths. So the current joke among women scientists is that we should care more about the length of the vasopressin gene in our mates than about the length of anything else. Maybe someday there will be a drugstore test kit—similar to a pregnancy test—for how long this gene is, so you can be sure you’re getting the best guy before you commit. Male monogamy may therefore be somewhat predetermined for each individual and passed down genetically to the next generation. It may be that devoted fathers and faithful partners are born, not made or shaped by a father’s example.

Our two closest primate cousins—chimpanzees and bonobos—also have different lengths of this gene, which match their social behaviors. Chimpanzees, who have the shorter gene, live in territorially based societies controlled by males who make frequent, fatal war raids on neighboring troops. Bonobos are run by female hierarchies and seal every social interaction with a bit of sexual rubbing. They are exceptionally social and have the long version of the gene. The human version of the gene is more like the bonobo gene. It would seem that those with the longer gene are more socially responsive. For example, this gene is shorter in humans with autism—a condition of profound social deficit. Differences in partner commitment behavior may therefore be related to our individual differences in the length of this gene and in hormones.