Scientists have known for more than 50 years that a hormone called oxytocin plays a critical role in stimulating uterine contractions during labor and delivery, and that afterward, it helps a nursing mother to release milk for her infant.
Men also produce oxytocin, it turns out, although at lower levels than women. Released during sexual arousal, it appears to promote feelings of contentment and attachment in both sexes, which accounts for one of its cuter nicknames: "the cuddle hormone."
But these days, scientists know oxytocin does so much more.
Made in a region of the brain known as the hypothalamus and secreted through the pituitary gland, the molecule reaches receptors throughout the body, modulating our moods and our social interactions in profound ways that suggest we are hardwired for empathy.
When experimenters at Claremont Graduate University administered oxytocin to male test subjects, for example, the men consistently scored higher on tests that measured pro-social traits like generosity and trust. At the same time, disruptions in the oxytocin system have been linked to autism, a disorder that is characterized by difficulty in forming human bonds.
Meanwhile, researchers from the University of California, Berkeley found that people who inherit variants of the gene that produces oxytocin receptors perform quite differently from one another on a battery of tests that measure how empathic and stress-prone they are.
They also knew that at a particular spot within the oxytocin receptor gene, people have one of three combinations of adenine (A) or guanine (G) molecules, which help form the base pairs in its DNA. Previous research has shown that people with the AA or AG variants are more likely to have had a childhood diagnosis of autism than those with the GG version.
Wondering whether these variations might reveal differences in people who have not been diagnosed with autism, they recruited 192 Berkeley college students and tested DNA drawn from their saliva samples. About 26 percent had the GG version, while the rest were either AA or AG, Rodrigues says.
Then they subjected the students to an empathy test called the Reading the Mind in the Eyes Task, showing them black-and-white photos of 36 strangers and asking them to judge which of four emotions they might be feeling. According to their paper published in the Proceedings of the National Academy of Sciences, members of the GG group were significantly more accurate than the rest while also rating themselves as more empathic in a self-report measure.
Two other tests to assess physiological and self-reported stress reactivity showed the GG group were calmer than the others.
The students were also asked about their parents' emotional style as well as their own interactions in romantic relationships to help account for differences in the nurturing styles that might have helped sculpt their empathic traits.
"We're definitely not trying to claim that that's not part of the equation," says Rodrigues, who is now an assistant professor of psychology at Oregon State University. "Genetics make just a small contribution to the whole person that we become."
The researchers also tested their own DNA as part of a pilot study. They were chagrined to find that none of them belonged to the more-empathic GG group, says Rodrigues, who declined to say whether she is an AG or AA.
"I really pride myself on being really pro-social, really empathic and pretty calm," she says. "I was a little disappointed, but I think it really helped me digest and make sense out of this stuff. ... It allowed me to do a lot of introspection about my family and how people have gone on different trajectories in life."
One limitation of the study is that the gene locus where the adenine-guanine variation occurs does not actually play a role in shaping the oxytocin receptors, but it is likely associated with active places in the gene that do, Rodrigues says.
Scientists also do not yet understand the different versions of the receptor on a molecular level, she says.
Given its effects on emotional functioning, it's not surprising that oxytocin appears to shape human social behavior, affecting our predisposition to follow the law, trust one another and reciprocate in economic transactions.
Zak and his team have male test subjects inhale a dose of pitocin (a synthetic form of oxytocin) before taking part in a computer game where they are asked to share some money with an unseen stranger. The enchanced-oxytocin group turns out to be a lot more generous than the controls.
Zak also runs similar experiments with males who received a boost of testosterone from a gel applied to their skin. They turned out to be less generous and more punitive than men with normal testosterone levels. Interestingly, Zak found the male tendency to enforce rules nonetheless tended to support sharing and cooperation on a group level.
"Oxytocin and testosterone really work on these evolutionarily old areas of the brain and the peripheral nervous system — the gut and the heart — that tell us whether it feels right or not," Zak says.
Research into oxytocin's behavioral effects languished for decades because researchers dismissed it as "a female hormone," Zak says. More recently, it became clear that in socially monogamous mammals, males and females have oxytocin receptors in their forebrains that help form what Zak has dubbed the HOME system (for Human Oxytocin-Mediated Empathy Circuit).
This feedback loop leads to the release of neurotransmitters that promote bonding and reduce anxiety, Zak says. "It essentially looks like we have this socially monogamous brain," he says.
"It is amazing that this little, ancient chemical that allows mammals to produce live births and to breastfeed spikes when someone sends you money by computer," Zak says.
"That's because it's a very robust mechanism. For 90 percent of the population it's very robust."
But some people seem to be immune to the effects of oxytocin, Zak says.
"They are people who are unconditional non-reciprocators," he says. "We call them 'bastards.'
"These are people who do not reciprocate when you're nice to them. They have very unusual personality traits. For example, they have a lot of sexual partners. What does that tell you? Well, they're not attaching to one person at a time."
The key of insight of all this research, according to Zak, is that "trust is kind of this economic lubricant. When trust is high, morale is high. ... Higher trust environments produce individuals who are happier."
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