The Chemistry of Commitment 

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

Ah, courtship. A northern harrier hawk repeatedly climbs the sky and then plunges toward the earth, enticing his desired partner to join him in this expression of untrammeled power. The humpback whale casts his mysterious and ever-changing song through miles of pelagic water, hoping it will be heard somewhere in the deep. A teenage boy anoints himself with a potent mix of styrene acrylate copolymer, hydrofluorocarbon, and fragrance — he calls it body spray — before heading to the mall. Humans have the most highly developed brains on the planet. Yet when it comes to sexual attraction and mate seeking, we're no different from our butt-sniffing animal cousins. Our passions are ruled by the same neurochemicals.

But love — that's a different story. Fundamentally, we are made for deep, lifelong love for one mate. Humans are among just 3 percent or so of mammals that seem to be hardwired for monogamy. This small set of monogamous mammals — including the prairie vole, the titi monkey, and the fat-tailed dwarf lemur — have a unique receptivity to oxytocin, the brain chemical that enables us to form the deep bond we know as love.

Most male animals mate early and often, with as many partners as possible. This system has reproductive advantage for the male of the species: The more eggs he can fertilize, the better the chances that some of his offspring will live to reproduce, passing along his genes.

There's a famous and probably apocryphal story about President Calvin Coolidge and his wife, Grace Anna. As the tale goes, the Coolidges visited a farm, where, on separate tours, they were each impressed by the amorous prowess of the top rooster. Mrs. Coolidge and her guide paused by the chicken coop, and she asked him how often the rooster copulated.

"Dozens of times a day," she was told.

"Well," she said, "please tell that to Mr. Coolidge."

Mr. Coolidge was duly told and, after a moment of dismay, he asked, "The same hen every time?"

"Oh, no, Mr. President. A different hen every time."

"Well," he said, "you tell that to Mrs. Coolidge."

Psychologists G. Bermant and D.F. Lott used this anecdote to explain a phenomenon they'd observed: A male rat who'd copulated to exhaustion with one particular female could somehow manage to get it up and begin all over again when presented with a different female rat in heat. The Coolidge effect is reliable not only in rats but also in most other mammals. It's the reason ranchers need only a single bull to service a herd of cows. But a bull will refuse to copulate again with a cow he's already mounted, no matter how they try to disguise her. When he's done, she's done.

Some women might swear that modern men experience the Coolidge effect. But men are not rats, hamsters, or bulls. While the brains of those creatures impel the males to spread their semen as widely as possible, the brain of the human male, like that of the human female, drives him to pin his desires on one particular woman. Each time he makes love, his orgasm teaches his brain that she, this one particular woman, is the source of pleasure and comfort. He can make love to her day after day after day until their grandkids present them with a gold-plated cookie platter.

Monogamy also has its own adaptive advantages. Instead of expending all their energy seeking mates — and exposing themselves to danger from rivals and predators — monogamous male mammals cohabit with their mates and make a fairly equal contribution to child rearing by gathering food, defending the nest, and making sure the offspring don't wander away. A man's investment in the offspring increases the likelihood that babies will grow up and carry on his genes, bettering his reproductive odds just as much as the sperm-scattering approach does. It's also a better deal for the female; her mate's protection and help make it more likely that she'll survive the perils of pregnancy, birth, and nursing.

For years, all of this was theoretical when it came to humans — the province of sociology and anthropology. But recent advances in brain scanning technology have enabled scientists to begin mapping the brain's circuits for fear, anxiety, and love. Functional magnetic resonance imaging, or fMRI, shows what parts of the brain become active as we do certain tasks. In an fMRI study, the subject may be told to do arithmetic, look at photographs, or think about someone he loves as he lies very still with his head inside the scanner. Patterns of increased activity show as bright spots in the scan; some parts of the brain "light up."

FMRI studies have identified the areas of the brain that consistently light up when people look at photos of loved ones. These are the same areas that come alive when mothers look at photos of their babies, and when they masturbate or make love. Moreover, when test subjects inhale oxytocin before a brain scan, activation in these areas increases. At the same time, oxytocin decreases activities in the parts of the brain that handle fear or anxiety. In other words, oxytocin increases our receptivity to connecting with others.

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