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Brain trust.
The hormone oxytocin may spur us to trust others even when they betray us by suppressing activity in the dorsal striatum (top, red regions) and amygdala (bottom).

Thomas Baumgartner/University of Zürich

The Neurochemistry of Forgiving and Forgetting

Trust forms the foundation of healthy relationships, and now scientists are zeroing in on how the feeling is triggered by chemicals in the brain. A new study shows that the hormone oxytocin may spur us to trust others even after they have betrayed us by suppressing a region of the brain that signals fear. The findings could lead to a better understanding of social phobias and related disorders.

Previous research has shown that oxytocin increases our feelings of trust and plays an important role in bonding with others (ScienceNOW, 1 June 2005). But the areas of the brain it acts on to produce that effect have remained a mystery. To get a better handle on how the hormone affects our noggins, Thomas Baumgartner, a neuroscientist at the University of Zürich, Switzerland, and colleagues monitored the brain activity of 49 men while they engaged in a game involving trust and betrayal.

In the game, the men were given money that they could share with another person who might increase the funds through investments and split the profits or betray them and keep all the money. When volunteers got a whiff of oxytocin via a nasal spray, their trust did not diminish even when the second player kept the money to himself half the time. In contrast, men who received a placebo spray reduced the amount of money they forked over.

Functional magnetic resonance imaging of the brains of the men who received oxytocin showed a drop in activity in a region called the amygdala, which plays a key role in triggering fear. The dorsal striatum, a region involved in learning from mistakes, also showed diminished activity. However, oxytocin did not reduce activity in these brain regions when the men played a similar game with a computer, confirming that the interaction with another person--and not just the hormone--was required to spur the changes, the researchers report in the 22 May issue of Neuron.

The findings suggest that oxytocin helps us maintain relationships by decreasing our fear of betrayal and other potential negative consequences of interacting with others, says Mauricio Delgado, a cognitive neuroscientist at Rutgers University in Newark, New Jersey. "Humans are typically averse towards social risks, so a little bit of oxytocin may facilitate carrying on relationships with others," he says.

The findings raise the possibility that social phobia is caused in part by a defect in how oxytocin normally regulates brain activity, Baumgartner says. Andreas Meyer-Lindenberg, a neuroscientist at the Central Institute of Mental Health in Mannheim, Germany, suggests that oxytocin signaling could also be disrupted in other disorders in which lack of trust or social attachment is a prominent problem, such as autism and schizophrenia.

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