Neurobiology of trust


Published: 24-07-2015

Last modified: 29-03-2020

The main role in trust plays oxytocin – a neurotransmitter and a hormone that also has an impact on stimulating the secretion of milk and starting labor. The role of oxytocin examined Paul J. Zak – professor of economics and clinical neurology, who after examining animal studies found that this molecule may play a role in the formation of trust in humans.

Animal studies

The research shows that in some mammals, oxytocin facilitates the interaction of individuals and vasotocin – a derivative of oxytocin – plays such a role also in other animals. According to evolutionary biologists vasotocin appeared approx. 100 million years ago in fish and was intended to weaken in female during ovulation natural fear of males and thereby facilitate reproduction. Vasotocin evolved to oxytocin and arginine vasopressin that occur in mammals. In the late 70s the results of studies in rodents have shown that oxytocin induces caring behavior in females. Other studies have shown that oxytocin and arginine vasopressin play a role in social behavior in voles. Prairie voles have one lifetime partner, co-create social groups and care for offspring, and the mountain voles are polygamists not caring for their offspring. Many scholars claim that for differences in social behavior of these two species is responsible a different location of oxytocin and arginine vasopressin receptors in their brains. In prairie voles those receptors are concentrated in areas of the midbrain that regulate the release of dopamine, which cause monogamy satisfying for this species.

Trust game

Paul J. Zak and his colleagues to examine the level of trust in humans have used the so-called. “Trust game”. They joined randomly tested subjects in pairs, denoted them as a player 1 and player 2, and gave a certain amount of money, eg. 10 dollars. Player 1 could give the player 2 a certain amount of money which were tripled (that is, if, for example. player 1 gave $ 6, the player 2 had on the account 6 x 3 + 10 = $ 28). Player 2 could return any amount to player 1. Players did not see each other, and all communications was made through computers. Immediately after the players made a decision, researchers collected blood samples from them to determine the level of oxytocin. The game was conducted in different countries and at different rates. Transfer made by the player 1 is considered the level of trust to the other person, and performed by the player 2 – a level of trustworthiness. It turned out that approx. 85% of the players 1 have transfered any amount to their partner, and 98% of players 2 have returned the player 1 some amount. Zak suspected that trust showed by a player 1 to player 2, caused increase in his levels of oxytocin, which should be the greater, the larger the amount was given to him.

Research of oxytocin levels confirmed these suspicions. In control studies, in which the money were passed randomly, it showed no increase in the level of oxytocin, so it is certain that the level has increased as a result of reliance, and not, for example because of satisfaction with enrichment. The researchers also found that higher levels of oxytocin in the players’ 2 blood was associated with increased trustworthiness – receiving a signal of trust from another person resulted in the transfer to him larger sums of money. This is probably due to the action of dopamine, which induces a feeling of satisfaction. The study of the brain using magnetic resonance imaging showed that during reciprocate of the trust are activated precisely those areas of the midbrain where dopamine is associated.

There was no evidence of oxytocin increase in the level of players 1 or correlation between the output level of this hormone in the players 1 and a sum of money transferred to the other player. It turned out, however, that increasing levels of oxytocin induce a greater tendency to trust. Those who got during the game a dose of oxytocin using nasal spray transfered to the player 2 by an average of 17% more money, and the percentage of players giving all its money increased from 25% to 50%. This demonstrates that increasing levels of oxytocin in the brain caused some people weakening of natural fear of interactions with strangers.

Studies also pointed to the differences in men and women in response to give a lack of trust. Most of the men, who the player 1 has given a small amount of money in a rematch did not pay anything, but most women have not taken vengeance and always repaid more or less constant part of what they have received. In the male body, who the player 1 did not show trust occurred an increase in dihydrotestosterone (DHT) – a derivative of testosterone, which is responsible for the physical changes that occur during puberty men and a propensity for aggression.

It is interesting that in those 2% of the players who did not pay anything to the player 1 researchers found very high levels of oxytocin. These people were also found personality traits similar to those that are characteristic of sociopaths, who have pleasure of other people’s suffering or are neutral to suffering. Probably oxytocin receptors of such persons are located in areas of the brain, which do not control the release of dopamine or are unregulated, that is do not respond to the signal of released oxytocin. The same is suspected in patients with autism, because they have low oxytocin levels, but increase its level does not increase their social engagement.


Paul J. Zak „Neurobiologia zaufania”, Świat Nauki, nr 07/2008

Author: Maja Kochanowska

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