The Trust Hormone

A 2005 study in Nature, for example, found that participants who received oxytocin were more likely to trust other people than participants who received a placebo. The recent Neuron study expands on the earlier experiment, showing that not only are people more trusting, but they are more trusting even when they know they are likely to be betrayed. The study also combines brain imaging data with the behavioral data to see which regions of the brain are affected by oxytocin.

“This was the first study where we combined brain imaging, social behavior and substance administration,” says Heinrichs.

Heinrichs and his colleagues performed the experiment on 49 male volunteers. Half of them received a nasal spray of oxytocin and half did not. Then each group played a trust game. The men had to decide whether or not to invest money with another volunteer in hopes of gaining more money. The amount they chose to invest would automatically be doubled. Then the volunteer receiving the money could decide how much, if any, to give back to the investor. The more money the volunteer invested, the more he stood to gain or lose.

The two groups played a round of six games and afterwards were told how often their investments paid off. About half the time, the trustee betrayed the investor. Each group then played another round of trust games with different trustees, and in this round the investors in the placebo group invested less often. But the oxytocin group continued to invest the same amount of money with the trustees.

After learning that their trust had been betrayed, the placebo group’s average investment dropped from about 7.7 to 6.5 out of a possible 12, while the oxytocin group’s average stayed about the same.

All participants were hooked up to a scanner while playing the trust games so their brain activity could be monitored. Researchers found that the amygdala was much more active in the placebo group than in the oxytocin group.

“The amygdala is very clearly identified with fear and anxiety,” says Ron Stoop of the Center for Psychiatric Neuroscience at the University of Lausanne, Switzerland. “When it is activated you get a fear response; when inactivated, people get very calm.”

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Oxytocin binds to proteins embedded in the cell membrane of the amygdala. This binding activates neurotransmitters that inhibit cells in the amygdala, reducing neural activity. The exact length of oxytocin’s inhibitory effect is unknown, but Stoop says it appears to only be short-lived.

Researchers are now trying to figure out whether oxytocin affects other areas of the brain, says Stoop. Oxytocin could be specific to the amygdala or it could have a general effect on the entire brain, he adds, noting that there are receptors in other parts of the brain like the hippocampus, where memories are formed, and the stria terminalis, which is a band of fibers that connects the amygdala to the hippocampus.

Oxytocin could eventually be used to treat social disorders characterized by excessive fear, such as social phobia, autism and post-traumatic stress disorder. Heinrichs says that the feeling of fear in these disorders is caused by a hyperactive amygdala. He thinks that oxytocin could help reduce that hyperactivity.

But Paul Zak, a neuroeconomist at Claremont University who studies how neural activity influences economic decisions, says it is premature to speculate on clinical uses of oxytocin. The study only evaluated its effects on men, and since oxytocin has a role in breastfeeding and childbirth, it would be important to determine whether the hormone has a different effect on women, he says.

Zak was also critical of the neural imaging used in the Neuron study. One effect of oxytocin is that it lowers a person’s heart rate. Since imaging shows brain activity based on the amount of blood flow, the activity in the amygdala might be lower not because it is being inhibited by oxytocin, but simply because there was less blood flow to the brain in general, according to Zak.