The discovery could lead to treatments for social phobia
A nasal spray which increases our trust for strangers is showing promise as a treatment for social phobia, say scientists from Zurich University.
They found that people who inhaled the "love hormone" oxytocin continued to trust strangers with their money - even after they were betrayed.
Brain scans showed the hormone lowered activity in the amygdala - a region which is overactive in social phobics.
Drug trials are under way and early signs are promising say the scientists.
Nicknamed the "cuddle chemical", oxytocin is a naturally produced hormone, which has been shown to play a role in social relations, maternal bonding, and also in sex.
Lead researcher Dr Thomas Baumgartner said: "We now know for the first time what exactly is going on in the brain when oxytocin increases trust.
"We found that oxytocin has a very specific effect in social situations. It seems to diminish our fears.
"Based on our results, we can now conclude that a lack of oxytocin is at least one of the causes for the fear experienced by social phobics.
"We hope and indeed we expect that we can improve their sociability by administering oxytocin."
Previous studies have shown that participants in "trust games" took greater risks with their money after inhaling the hormone via a nasal spray.
In this latest experiment, published in the journal Neuron, the researchers asked volunteer subjects to take part in a similar game.
They were each asked to contribute money to a human trustee, with the understanding that the trustee would invest the money and decide whether to return the profits, or betray the subject's trust by keeping the profit.
The subjects also received doses of oxytocin or a placebo via a nasal spray.
After investing, the participants were given feedback on the trustees. When their trust was abused, the placebo group became less willing to invest. But the players who had been given oxytocin continued to trust their money with a broker.
"We can see that oxytocin has a very powerful effect," said Dr Baumgartner.
"The subjects who received oxytocin demonstrated no change in their trust behaviour, even though they were informed that their trust was not honoured in roughly 50% of cases."
In a second game, where the human trustees were replaced by a computer which gave random returns, the hormone made no difference to the players' investment behaviour.
"It appears that oxytocin affects social responses specifically related to trust," Dr Baumgartner said.
During the games, the players' brains were scanned using functional magnetic resonance imaging (fMRI).
The researchers found that oxytocin reduced activity in two regions which act as natural "defence barriers".
They are the amygdala, which processes fear and danger, and an area of the striatum, which helps to guide future behaviour, based on reward feedback.
The amygdala has been found to be extremely active in the brains of sufferers of social phobia.
Dr Baumgartner's colleague, Professor Markus Heinrichs, has begun a study where social phobia sufferers are given either oxytocin or a placebo, in combination with cognitive and behavioural therapy.
The trials are ongoing, but Dr Baumgartner said that early signs appear "promising".
The hormone could also be a candidate for treating patients with autism, he says.
"Autistic people also have a fear of social situations and have problems interacting, so it is very likely that oxytocin could help," he said.
"This hormone seems to play a very specific role in social situations so might be able to improve autism. But so far I am not aware of any studies."
Mauricio Delgado, a psychologist at Rutgers University, said: "This study has significant implications for understanding mental disorders where deficits in social behaviour are observed.
"While a degree of wariness may protect one from harm, being able to ''forgive and forget'' is an imperative step in maintaining long-term relationships.
"The reported oxytocin finding could provide a bridge for potential clinical applications."