Monthly Archives: July 2008

Mysteries of the Brain Part – 18 – TRUST


In this episode of the series we explore how and why we trust other people. What brain processes are involved in the social interaction of trust? Is there a hormone that is responsible for trust? We will study all this today. This VERITAS article is based on a Scientific American magazine article in the June edition of the magazine.

Human society is what it is today because we tend to trust each other- we even trust the people we do not know. If you go to a new city and ask for directions you are reasonably certain that the directions given by strangers on the street will be correct. We tend to trust each other except
in some very specific situations.

All animals, even less developed ones like fish and lizards display trust in atleast one interaction: sex. All sexually reproducing animals display this trust. The females of any species will let a male approach her when she is ovulating because the benefits of sex( offspring and genetic diversity) overweigh the danger of being harmed( or even eaten) by the male of the species. Scientists have found that in fish the release of
the hormone vasotocin reduces the female’s fear of being approached by a male when she is ovulation. Thus the trust displayed by a female fish on a male fish for sex is due to the hormone vasotocin.

In mammals the hormone oxytocin plays a huge role in female reproduction: from sex to labor to lactation. And Scientists have found that this hormone is responsible for all social interactions involving trust. So the complex trust that we humans exhibit on each other seems to have evolved from the trust that even simple creatures display during sex. And this trust is also due to a hormone in the blood-oxytocin.

To test their theory about the role of oxytocin in trust Scientists devised a “trust game”. They asked people to volunteer promising them 10$ if they play the game. In any game session two people played the game: let’s call them A and B. A and B did not know each other and they do not meet during or after the game. The game gives A 10$ and B 10$. Now A can give B some of his 10$. Let’s say he gives B x dollars. If that
happens then B will get his 10$ plus 3 times the amount that A gives him. So if A gives B 6 dollars then B gets 10$ + 6 X 3 = 28$. Now B can return some of the money that he made to A to thank him for making that extra money. But if A does not want to give any money to B he is not forced to do it- he can display a non-trusting behavior: what happens if I give money to B and he makes the extra money but does not give me
back any of it. Similarly B can act selfish: he can keep all the money without giving anything back to A.

The Scientists played this game with several sets of players and found that 85% of the ‘A’ people gave some of their money to B. And 98% of the “B” players went on to return some of their money to A. The researchers found that B players with higher levels of oxytocin returned higher amounts of money to the A players. These B players responded more positively to the trust placed on them by their A counterparts.

The researchers also calculated the change in oxytocin levels in B subjects when they got some money from A. If the A player sent more money to B the oxytocin level in the B players increased more compared to when less money was sent by A. So your oxytocin level increases when someone displays trust in you and since now you have more oxytocin level you tend to respond to the trust in a more positive way: you act more trustworthy!

And then the researchers went on to consider how our hormone levels change when someone displays distrust in us. And here there is a difference between males and females. If during the trust game a player A did not give any money or very little money to a male player B his(B’s) DTH( dihydrotestosterone) levels would increase. DTH is responsible for violent  behaviour in males. So when a male is distrusted he tends to exhibit a violent behavior.  However if a female B player did not get any money from A she would simply not return any. And if she received a small amount from A she will return a small amount back. There will be no change in her DTH levels but she does respond with : ” you gave me little so I will return little” approach.

The article says that our oxytocin levels may vary minute by minute and over our entire lifespan. And life experiences may retune the base oxytocin levels to a different level thus making us generally more trusting or less trusting.

This indeed is a very interesting study and something that paves the way for treating the hatred and distrust that exists in our society for people who follow different religions or are from different societies or countries. And one thing is for certain: if you place trust in someone his desire to become trustworthy increases. This is the moral of the story for me.



Go, wondrous creature! mount where Science guides:
Go, measure earth, weigh air, and state the tides:
Instruct the planets in what orbs to run,
Correct old time and regulate the Sun;