Receiving cash boosts oxytocin levels. Credit: © Corbis

Trust begets trust - and the hormone oxytocin, research reveals. The chemical messenger may help humans to bond, researchers told this week's Society for Neuroscience Annual Meeting in New Orleans, Louisiana.

People's oxytocin levels rise when they receive a signal of trust, says Paul Zak from Claremont Graduate University in California. Those with the highest hormone levels are more likely to be generous in return and so are more trusting, he says.

Zak's team gave 19 people $10 apiece. Each person was invited to share their reward with an anonymous recipient. The recipients' money was then tripled and they were allowed to send a share back to the donors. The researchers found that 54% of recipients returned money to donors. Those who gave and returned most generously had the highest oxytocin levels.

Women in the study who were ovulating were less likely to be trusting, the team also found. This makes sense, says Zak. Females who could get pregnant need to protect their resources. And they need to be selective in their interpretation of social signals so as to choose the best mate, he reasons.

The study may also shed light on the causes of autism, says neuroscientist Richard Frackowiak from University College London: autistic people often trust too much. Oxytocin sends signals to cells in the amygdala - a brain region involved in emotion and social behaviour and implicated in autism.

Oxytocin prompts new mothers to release breast milk, strengthening the bond between mother and child. Touching boosts oxytocin levels in rats. The chemical also suppresses stress hormones.