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This body of research consistently shows that the human brain is wired like those of the monogamous 3 percent. And it shows that oxytocin is the brain chemical not only of bonding and maternal care in humans, too, but also of trust, generosity, and empathy.
Dacher Keltner, a UC Berkeley professor of psychology and director of the Berkeley Social Interaction Laboratory, says that because the bond between mates and their children is crucial for the survival of the species, "the capacity to be kind would be woven into the genetic fabric of this new hominid" during our evolutionary development.
As we evolved into monogamous, familial mammals, differences in the sexes would have been adaptive, allowing the mother and father to protect and nurture their offspring in compatible ways. And while our culture today allows for less rigidly defined gender roles, we're still working with the sexual chemistries of cave people. Humans aren't hard-wired for sexual behavior, nor for monogamy; indeed, there's wide variation among individuals. But understanding how our neurochemistry affects our lives and loves can help us make sense of those mystifying differences between men and women, whether we want to let them play out or escape from them.
Love in the Brain
Whether a species is monogamous seems to depend on a small variation in the distribution of oxytocin and dopamine receptors in the brain. Most mammals, male and female, monogamous or not, have remarkably similar brain structures and neurochemistry. Their brains produce oxytocin, as well as vasopressin, a closely related molecule that seems to stimulate protective behaviors. Dopamine, the chemical messenger responsible for anticipation of pleasure, drives them all to go after the rewards of food and sex.
What's different about the monogamous minority is the way their brains seem set up to pair the rewarding rush of dopamine with the neurochemical of social connection, oxytocin. The peculiar placement of oxytocin receptors in the brains of humans and the rest of the monogamous 3 percent ties social attachment to the brain's powerful reward structures. The key to whether a mammal will mate for life or run from one partner to the next seems to be where receptors for those neurotransmitters are located, according to Larry Young of Emory University and Zuoxin Wang, of the University of Florida. In monogamous brains, the reward centers are rich with receptors for not only dopamine but also oxytocin, the molecule of bonding. The same systems involved in addiction to drugs leads humans to form a benign addiction to our mates, families, and friends.
In the nonmonogamous brain, sex feels really good, while in the monogamous brain, the rewards of sex are tied to one particular sex partner, creating a conditioned response that's not much different from the addicting high of cocaine or methamphetamine. The combined chemical message says, "Sex felt really good with you. Let's do it some more."
Evolution may have hijacked the brain's reward circuits in aid of monogamous love, because raising weak and defenseless human children in pairs provided adaptive advantages. In prehistoric times (and even today) children were more likely to survive to pass on their parents' genes in a dual-parent family.
But the woman and children's need for support and defense conflicts with the sexual promiscuity that, in most species, creates reproductive success for the male. Monogamy helps resolve the conflict. A man who lives with his mate and has sex with her frequently is more likely to be the biological father of the offspring he's investing in. In primitive times, the male half of the pair could protect his woman and children from animal and human predators, and he could provide food when pregnancy, labor, or sickness kept her lying in. Those adaptive advantages seem to remain in our modern times. The largest predictors of divorce in the 20th century were infidelity and infertility, according to a 2000 study by Chris Fraley and Phillip Shaver of UC Davis.
This is not to say that men — or women — are so hardwired for monogamy that infidelity is impossible, or even difficult. Obviously, sex outside of marriage is just a phone call away. We humans, like most monogamous mammals, engage in what biologists call "social monogamy," defined as living with one partner to whom we're bonded, sharing the work of maintaining a home and raising children, with the occasional sexual adventure. In fact, there may be no such thing as true sexual monogamy anywhere in the animal kingdom.
Scientists can estimate how often pairs in a monogamous species copulate with other individuals by doing DNA tests on the young. The results show that monogamy is more of a social system than a sexual one. Researchers who've examined the DNA of monogamous birds in Europe have found that between 29 and 68 percent of the offspring are the result of "extra-pair copulation."
We don't seem to have any comparable stats for human paternity, but anecdotal reports suggest that copulation outside of monogamy is more frequent that people imagine. Surgeons who do organ transplants or bone marrow transfusions typically look first to parents or siblings as donors, because they share genetic material. However, in many more cases than they expect, when they do the DNA test, there turns out not to be a match: The father of the family wasn't the biological father of a daughter, or the brother and sister are really half siblings. It seems that modern humans are also likely to hedge their genetic bets with some extra-pair copulation.
Men and Women are Different
While oxytocin is the brain chemical of love, trust, and generosity in men and women, there's a joker in the deck: vasopressin. This somewhat mysterious chemical differs from oxytocin by just two amino acids; both chemicals probably evolved from a single precursor in the days when our ancestors slithered. Researchers haven't quite figured out what vasopressin's role is, but it may be the key to some of the most confounding differences in the way that men and women lust and love.
Vasopressin is much harder to study in humans than oxytocin is, for three reasons. First, the level in a person's bloodstream doesn't correlate as well with levels in the nervous system, so researchers can't rely on blood samples to understand what's going on in the brain. Second, you can't find people willing to take part in studies where vasopressin, or anything else, is injected into their brains. Third, scientists have identified three different types of vasopressin receptors in humans, the same as in prairie voles.
Studies of rodents, including mice, rats, and voles, indicate that vasopressin may alter a male's bond with its mate. Scientists used to think that vasopressin regulates male monogamy the same way oxytocin does the female's. But Karen Bales of UC Davis and Sue Carter of the University of Illinois have done experiments showing that it's oxytocin that's key for both male and female bonding. When Bales injected the brains of male prairie voles with a substance to block the effects of vasopressin, it had no effect on their bonds with their mates. After a period of separation, males whose vasopressin had been blocked scurried just as eagerly back to their chosen companions. In another experiment, Bales gave male prairie voles a single dose of an oxytocin blocker soon after they were born. When these males grew up, they failed to bond with a mate. They also displayed a lot less of the parental caring behavior typical of this species: They spent less time "huddling" with the pups, licked and groomed them less, and weren't as apt to retrieve babies that wandered out of the nest. These experiments show that oxytocin is a critical component of a male's bonding as well as a female's.
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