The Biology Behind the Milk of Human Kindness

As the festival of mandatory gratitude looms into view, allow me to offer a few suggestions on what, exactly, you should be thankful for.

Be thankful that, on at least one occasion, your mother did not fend off your father with a pair of nunchucks, but instead allowed enough contact to facilitate your happy conception. Be thankful that when you go to buy a pale, poultrylike entity, the grocery clerk will accept your credit card in good faith and even return it with a heroic garble of your last name. Be grateful for the empathetic employee working the United Airlines ticket counter the day after Thanksgiving, who understands why you must leave town today, this very minute, lest someone pull out the family nunchucks.

Above all, be thankful for your brain’s supply of oxytocin, the small, celebrated peptide hormone that, by the looks of it, helps lubricate our every prosocial exchange, the thousands of acts of kindness, kind-of kindness and not-as-nakedly-venal-as-I-could-have-been kindness that make human society possible. Scientists have long known that the hormone plays essential physiological roles during birth and lactation, and animal studies have shown that oxytocin can influence behavior too, prompting voles to cuddle up with their mates, for example, or to clean and comfort their pups. Now a raft of new research in humans suggests that oxytocin underlies the twin emotional pillars of civilized life, our capacity to feel empathy and trust.

Reporting this month in The Proceedings of the National Academy of Sciences, researchers found that genetic differences in people’s responsiveness to the effects of oxytocin were linked to their ability to read faces, infer the emotions of others, feel distress at others’ hardship and even to identify with characters in a novel or “Doonesbury.” “I came into this research as a big skeptic,” said Sarina M. Rodrigues of Oregon State University, an author of the new report, “but the results had me floored.”

Oxytocin may also be a capitalist tool. In a series of papers that appeared in Nature, Neuron and elsewhere, Ernst Fehr, director of the Institute for Empirical Research in Economics at the University of Zurich, and his colleagues showed that the hormone had a remarkable effect on the willingness of people to trust strangers with their money. In the Nature study, 58 healthy male students were given a single nasal squirt of either oxytocin or a placebo solution and, 50 minutes later, were instructed to start playing rounds of the Trust Game with each other, using monetary units they could either invest or withhold.

The researchers found that the oxytocin-enhanced subjects were significantly more likely than the placebo players to trust their financial partners: whereas 45 percent of the oxytocin group agreed to invest the maximum amount of money possible, just 21 percent of the control group proved so amenable. Moreover, the researchers showed that the oxytocin boost didn’t simply make subjects more willing to take risks and throw their money around. When participants knew they were playing against a computer rather than a human being, there was no difference in investment strategy between the groups. Trust, it seems, is a strictly wetware affair.

Yet the hormone doesn’t turn you into a sucker. In the Nov. 1 issue of Biological Psychiatry, Simone Shamay-Tsoory of the University of Haifa and her colleagues reported that when participants in a game of chance were pitted against a player they considered arrogant, a nasal spritz of oxytocin augmented their feelings both of envy whenever the haughty one won and of schadenfreudian gloating when their opponent lost.

As a rule, though, oxytocin is a joiner not a splitter. Analogues of the molecule are found in fish, perhaps to help facilitate the delicate business of fertilization, by inhibiting a fish’s natural tendency to flee from other fish. The more elaborate grew the social demands, the more roles oxytocin assumed, reaching its apotheosis in mammals. If you’re going to give birth to a litter of needy young, why not let the same signal that helped push those mewlers into the world give you tips on their care and feeding? And if you’re a human, bent on turning everything into an extended family affair, here is oxytocin again to cheerlead and teleprompt. C. Sue Carter of the University of Illinois at Chicago, a pioneer in the study of oxytocin, suspects that the association between the hormone and childbirth long kept scientists from taking it seriously. “But now that it’s been brought into the world of economics and finance,” Dr. Carter said, “suddenly it’s very hot.”

Oxytocin acts as a hormone, traveling through the bloodstream to affect organs far from its origin in the brain, and as a kind of neurotransmitter, allowing brain cells to communicate. Unlike most neurotransmitters, oxytocin seems to deliver its signal through just one receptor, one protein designed to recognize its shape and shudder accordingly when clasped; dopamine and serotonin, by contrast, each have five or more receptors assigned to their care. Yet the precise contours of oxytocin’s hardworking receptor differ among individuals, to apparently noticeable effect.

In their new study, Dr. Rodrigues and Laura R. Saslow and Dacher Keltner of the University of California, Berkeley, looked at how two variants in the genetic code for the receptor might influence a person’s capacity for empathy, as measured by a standard empathy questionnaire (“I really get involved with the feelings of the characters in a novel”) and a behavioral task called “Reading the Mind in the Eyes.” In it, participants looked at 36 black-and-white photographs of people’s eyes and were asked to choose the word that best described each subject’s mood. Uneasy, defiant, contemplative, playful? In a related measure of oxytocin’s presumed calming effects, subjects were also tested for how strongly they reacted to the stress of hearing a series of loud noises.

In their sample of 192 male and female college students, the researchers found that those carrying the so-called A version of the oxytocin receptor, which previous reports had associated with autism and poor parenting skills, scored significantly lower on the eye-reading task and higher on the stress-prone test than did subjects with the G variant of the receptor.

“We’re all different, and that’s a good thing,” Dr. Rodrigues said. “If everyone were gooey and lovey-dovey, it would be an obnoxious world.” As she drolly admitted, she herself is Type A.