(4 of 5)
When Mating Becomes Love
If we've succeeded in becoming such efficient reproductive machines—equipped with both a generous appetite for mates and a cool ability to screen them for genetic qualities—why muddy things up with romance? For one thing, we may not be able to help it. Just being attracted to someone doesn't mean that that person is attracted back, and few things drive us crazier than wanting something we may not get. Cultural customs that warn against sex on the first date may have emerged for such practical reasons as avoiding pregnancy or sexually transmitted diseases, but they're also there for tactical reasons. Males or females who volunteer their babymaking services too freely may not be offering up very valuable genes. Those who seem more discerning are likelier to be holding a winning genetic hand—and are in a better position to demand one in return.
The elaborate ritual of dating is how this screening takes place. It's when that process pays off—when you finally feel you've found the right person—that the true-love thrill hits, and studies of the brain with functional magnetic resonance imagers (fMRIs) show why it feels so good. The earliest fMRIs of brains in love were taken in 2000, and they revealed that the sensation of romance is processed in three areas. The first is the ventral tegmental, a clump of tissue in the brain's lower regions, which is the body's central refinery for dopamine. Dopamine does a lot of jobs, but the thing we notice most is that it regulates reward. When you win a hand of poker, it's a dopamine jolt that's responsible for the thrill that follows. When you look forward to a big meal or expect a big raise, it's a steady flow of dopamine that makes the anticipation such a pleasure.
Fisher and her colleagues have conducted recent fMRI scans of people who are not just in love but newly in love and have found that their ventral tegmental areas are working particularly hard. "This little factory near the base of the brain is sending dopamine to higher regions," she says. "It creates craving, motivation, goal-oriented behavior—and ecstasy."
When Love Becomes a Habit
Even with its intoxicating supply of dopamine, the ventral tegmental couldn't do the love job on its own. Most people eventually do leave the poker game or the dinner table, after all. Something has to turn the exhilaration of a new partner into what can approach an obsession, and that something is the brain's nucleus accumbens, located slightly higher and farther forward than the ventral tegmental. Thrill signals that start in the lower brain are processed in the nucleus accumbens via not just dopamine but also serotonin and, importantly, oxytocin. If ever there was a substance designed to bind, it's oxytocin.
New mothers are flooded with the stuff during labor and nursing—one reason they connect so ferociously to their babies before they know them as anything more than a squirmy body and a hungry mouth. Live-in fathers whose partners are pregnant experience elevated oxytocin too, a good thing if they're going to stick around through months of gestation and years of child-rearing. So powerful is oxytocin that a stranger who merely walks into its line of fire can suddenly seem appealing.
"In one study, an aide who was not involved with the birth of a baby would stand in a hospital room while the mother was in labor," says Sue Carter, a professor of psychiatry at the University of Illinois. "The mothers later reported that they found the person very sympathetic, even though she was doing nothing at all."
The last major stops for love signals in the brain are the caudate nuclei, a pair of structures on either side of the head, each about the size of a shrimp. It's here that patterns and mundane habits, such as knowing how to type and drive a car, are stored. Motor skills like those can be hard to lose, thanks to the caudate nuclei's indelible memory. Apply the same permanence to love, and it's no wonder that early passion can gel so quickly into enduring commitment. The idea that even one primal part of the brain is involved in processing love would be enough to make the feeling powerful. The fact that three are at work makes that powerful feeling consuming.