By Paul Zak

Trust pervades nearly every aspect of our daily lives, yet the neurobiological mechanisms that permit human beings to trust each are not understood.  In this research we find that when someone observes that another person trusts them, a hormone called oxytocin that circulates in the brain and the body rises. The stronger the signal of trust, the more oxytocin increases.  In addition, the more oxytocin increases, the more trustworthy (reciprocating trust) people are.  Interestingly, participants in this experiment were unable to articulate why they behaved they way they did, but nonetheless their brains guided them to behave in “socially desirable” ways, that is, to be trustworthy.  This tells us that human beings are exquisitely attuned to interpreting and responding to social signals. 

Our findings are even more surprising because monetary transfers were used to gauge trust and trustworthiness, and the entire interaction took place by computer without any face to face communication.  Signals of trust are sent by sending money that participants earned to another person in a laboratory, without knowing who that person is or what they will do.  That, is, there is a real cost to signaling that you trust someone.  This research is part of a new transdisciplinary field called “neuroeconomics” that measures the neurologic processes involved in decisions involving money. 

This is how the experiment works: People were recruited and paid $10 for showing up.  Then they took seats in a large computer lab and were matched up in pairs, but this done completely anonymously so that no one knew (or would know) the other person in his or her pair.  One-half of the participants (decision-maker 1s) then had the opportunity to send none, some or all of their $10 show-up fee to the other person in their pair.  Whatever is sent is tripled.  So, if $4 was sent, the other person would have $22 ($4 tripled, plus the $10 show-up fee the second person receives).  The second decision-maker could then send some amount of this money back to decision-maker 1, but need not.  This is how we produce a social signal of trust: decision-maker 1’s only reason to transfer money to the other person is because he or she trusts that that person will understand why the money is being sent to them, and in turn will return some to them (be trustworthy).  All subjects are told that the initial monetary transfer is tripled, and there is no deception of any kind.

After each person makes his or her decision, they are taken to another room and four tablespoons of blood were taken from an arm vein.  Animal studies have shown that oxytocin, a hormone little studied in humans, facilitates social recognition and social bonding, for example, bonding of mothers to their offspring, and in some monogamous species the bonding of males and females in a family unit.  Based on the animal studies, we hypothesized that what is happening in the trust experiment is that people are forming temporary social bonds with the other person in their pair.  This is just what we found. The stronger the signal of trust, the more oxytocin increases, and the more trustworthy people are.   This is surprising given the sterile laboratory environment of the interaction so that the effect of oxytocin on face-to-face interactions must be quite strong.

We also found that women in the experiment who are ovulating were significantly less likely to be trustworthy (for the same signal of trust).  This effect is caused by the physiologic interactions between progesterone and oxytocin, and it makes sense behaviorally: women who are, or are about to be, pregnant, need to be much more selective in their interpretation of social signals, and also need more resources than at other times. 

Standard economic theory (the “Nash equilibrium”) predicts that rational self-interested people should never trust another person, and if someone trusts you, you should not be trustworthy.  Why?  The Nash equilibrium says that if you are decision-maker 2, you should prefer more money than less so you should not be trustworthy (that is, return any money to decision-maker 1).  Decision-maker 1 should realize this and therefore never send anything to the second person.  Yet we see abundant trust in the lab and in daily life. 

What the Nash equilibrium ignores is that humans, while certainly self-interested, also are highly social creatures and have brains designed to interpret social signals; in other words, we care what others think about us and our brains motivate us to take others into account.  This could be called empathy.  There is little evidence that creatures besides humans are empathetic, and indeed humans are empathetic even to strangers.  This reveals an important role for the emotions in decision-making.  Further, such empathy enables unrelated humans to live together with generally little violence in large cities and makes modern industrial economies possible. 

My lab is now studying brain activation patterns when people receive signals of trust, as well as in the physiologic responses to trust signals in patients who have neurologic damage.  Trust is an essential part of our daily lives, from walking down the street to driving to countless other daily activities, so that discovering the neurobiology of trust tells us something important about human nature: that we are so highly social that we pick up social signals of trust and act on them even when we are not consciously aware of these signals.   Our brain acts as an internal compass that guides us towards the “right” thing to do.