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Zack Lynch is author of The Neuro Revolution: How Brain Science Is Changing Our World (St. Martin's Press, July 2009).
He is the founder and executive director of the Neurotechnology Industry Organization (NIO) and co-founder of NeuroInsights. He serves on the advisory boards of the McGovern Institute for Brain Research at MIT, the Center for Neuroeconomic Studies, Science Progress, and SocialText, a social software company. Please send newsworthy items or feedback - to Zack Lynch.
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August 11, 2003

The Neurobiology of Trust

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Posted by Paul

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.

Comments (5) | Category: Neuroeconomics


1. Zack Lynch on January 8, 2004 6:13 PM writes...

A good piece on involutary trust signals:

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2. Paul Zak on January 9, 2004 5:01 PM writes...

The recent papers Tyler Cowen discusses on the presumed transparency of involutary trust signals( identifies an interesting apsect of human social behavior. But Cowen suggests this is driven by "excess sympathy". The results from my lab don't quite agree with this: they show that humans processes signals of trust physiologically, and that this affects their subsequent behaviors, even without seeing or talking to those they are interacting with. I interpret this not as "excess sympathy" but as part of the mammalian social navigation architecture that permits us to be the social beings we are. Our results also show that some subjects recieve a strong trust signal, have a big physiologic response, but are not trustworthy in return. We are now studying these subjects to understand how they suppress the strong bias we find toward reciprocal cooperative behavior (and these subjects are not only economics students!). Economists have by and large ignored the (social, institutional, environmental) context in which economic transactions occur (for an exception showing the effects on economic growth, see Zak, P.J. and Knack, S., "Trust and Growth" The Economic Journal, 111:295-321, 2001).

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3. Majdi Haroun on January 11, 2004 11:49 AM writes...

I find your comments very interesting especially in the light of my personal experience interacting with eastern cultures for the past 20 yrs (lived most of my life in Kuwait, India, Egypt, Korea, to list a few). I have keenly observed how such incidents (subjects receiving a strong trust signal, have a big physiologic response, but are not trustworthy..). Such phenomenon is more evident in the east due to the fact that many of the daily interactions (including commerce, trading, etc...) rely entirely on the Trustworthiness of the parties. I found it especially very intriguing how some eastern communities have created widely accepted stereotypes of tribes/villages that are labeled as "cunning as a snake" or "trustworthy as an elephant".

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4. Paul J. Zak on January 13, 2004 10:11 AM writes...

Majdi Haroun has identified a key issue regarding the role of trust in econmic development. Because "men are not angels", institutions that enforce contracts are necessary to raise standards of living. These institutions include independent judiciaries, investigators, regulatory agencies, as well as informal social networks to enforce norms. To the extent these are missing, trading typically occurs among kin or ethnic groups, or as Mr. Haroun suggests, in those villages that are known to be more trustworthy. The problem with this is that, as Adam Smith observed in 1776, kin/ethnic trading limits the extent of the market generally does not raise living standards at the country level. There may be, though, a virtuous feeback mechanism where those in trustworthy villages internalize a norm of behavior and even outside of their home village are more trustworthy. The neural basis for social norms is an important goal for future research.

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5. Elizabeth Kunkel on September 13, 2004 12:12 AM writes...

Very interesting concepts regarding neurobiology of trust. Have any further studies or articles been done regarding this subject matter? I'm currently in law school and I think this would be a fascinating concept to study in regards to the influence of lawyers on juries. (or if no jury, then in front of judges). While the facts and evidence are important, I think people innately are driven to want to believe one person over another based on how they present themselves. Trustworthiness is a key component of credibility. If you can "fake" credibility, that's quite an accomplishment.

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