Small doses of the male sex hormone testosterone can make people less trusting of strangers, but the effect only seems to be true for the more trusting members of society, scientists have found.
Testosterone is produced by both sexes, although in much smaller quantities in women, and has been linked with aggression and competitive behaviour.
The researchers found that when testosterone is administered as a small, one-off dose to female volunteers, their sense of trust towards strangers changes, but only if the women tend to be trusting in the first place.
Jack van Honk of the University of Cape Town in South Africa said the findings suggested that testosterone generated mistrust in more gullible individuals as a way of protecting them against the deceitful behaviour of a competitive world.
He suggested that testosterone may work in opposition to the "love" hormone oxytocin, which is produced for instance in women during childbirth and promotes social bonding and trust between individuals.
The study, published in the journal Proceedings of the National Academy of Sciences, tested a panel of 24 young women for how trusting they felt towards a photographic gallery of strangers. They were then each given a small dose of testosterone under the tongue and asked to re-evaluate the photographic gallery.
The women who started out more trusting of strangers became significantly less trusting after taking the single shot of low-dose testosterone. No significant effect was seen in the women who were less trusting to begin with.
"We find the effect in socially naive people, those who are easily deceived by others. Maybe the hormone could help when the condition is pathological or damaging to the person," Dr van Honk said. Trust was crucial to the development of the earliest human societies where co-operation between unrelated individuals led to a better chance of survival. But this trust had to be counterbalanced with an awareness of possible cheating, he added.