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Biology 202, Spring 2005 Third Web Papers On Serendip

A Closer Look at the Familiar: What Do We Know About Trust?

Xuan-Shi, Lim

What does it mean to trust someone? Trust is often felt as an intuition or feeling that one has about others. We may trust someone consciously or unconsciously and may decide by assessing the way a person looks, talks, and behaves. Past experiences of betrayal may influence one's willingness to trust others in future interactions. I have attempted to construct a layman's understanding of trust that is related to social and psychological factors. In the age of neuroscience, several scientists have asked: what is going on in our brains as we allow ourselves to trust someone? The role our brain plays in the process of building trust has recently been investigated by King-Casas et al (2005), who reported that the development of trust between two individuals may be mapped onto a pattern of activity at the caudate nucleus (1). This study raises several interesting questions for me: What is trust and is it possible to understand trust from a neurological standpoint? Can trust be rebuilt after betrayal? This paper examines two scientific studies on trust in hopes of answering these questions.

In their experiment, King-Casas et al used event-related hyperscan-fMRI (2) to simultaneously examine the brain activity of two subjects as they played the trust game for ten rounds (1). The basic design of the game is as follows: one player is designated the investor and the other, the trustee. At the start of the game, the investor receives $20 and would decide how much of it to transfer to the trustee, keeping in mind that the amount of his or her investment would increase threefold under the trustee's charge (1). Upon receiving the tripled amount, the trustee would decide how much of it he or she wishes to repay the investor (1). After a player makes a decision, both parties are presented with the details of that transaction; their individual screens display the amount, in bar graphs and numbers, that the decision-maker has kept and has chosen to give away (1). A total of 48 pairs of subjects played the trust game and each pair was unaware of each other's identity (1). King-Casas et al claimed that they were studying trust at its core because their experimental design removed "the confounding elements of context and communication known to influence trust" (1). This study is the first to simultaneously monitor the brain activity of two individuals as they interact with each other (1).

King-Casas et al identified the caudate nucleus as the brain region that functions to build trust between two individuals. The brain scans of trustees revealed greater activity at the head of the caudate nucleus when the investor increased the sum of his or her previous investment in response to the trustee's decision to reduce the sum of repayment (1). There was decreased activity at the trustee's caudate nucleus when the investor made a smaller investment in response to the trustee's decision to increase the sum of repayment (1). In other words, activity at the trustee's caudate nucleus seems to be altered by both generosity and stinginess. It was also discovered that in later rounds of the trust game, the "intention to trust" signal appeared 14 seconds earlier at the trustee's caudate nucleus, when compared to early rounds when the signal only appeared after the investor's decision was revealed (1).

Given these findings, King-Casas et al suggested that the head of the caudate nucleus evaluates the fairness of the investor's decision and reveals whether the trustee intends to repay the investor's decision with trust (1). As the caudate nucleus receives dopamine inputs from other brain regions and dopamine has been implicated in the prediction of rewards, King-Casas et al speculated that the trustee is more willing to trust after his or her brain learns (and is thus able to predict) that the investor's response is likely to benefit the trustee (1).

I was slightly surprised to learn that the caudate nucleus may be the seat of trust because this region is usually associated with movement; it is part of the basal ganglia (3). In both Parkinson's and Huntington's diseases, the caudate nucleus is affected (3), thus resulting in the loss of voluntary control over movements. Symptoms of obsessive compulsive disorder are also associated with abnormal activity at the caudate nucleus (4). The nucleus accumbens, rather than the caudate nucleus, is usually associated with reward. Food, sex, nicotine, and cocaine, for instance, are rewarding because they increase dopamine action at the nucleus accumbens (4). Nevertheless, it is probable that the caudate nucleus is involved in reward prediction. I was somewhat bothered by the suggestion that our decision to trust someone may arise from a perhaps unconscious calculation of the probability of reward in that interaction. Am I mistaken to think that trust generally involves goodwill or a magnanimous faith in others?

While King-Casas et al have created a well-designed study and provided a reasonable interpretation of their findings, I wonder if they were investigating genuine trust. Firstly, the subjects were playing a monetary game that involved making decisions about whether to trust the other player. This study's findings may therefore apply only to trust that relates to financial decisions and may not be generalizable to other types of social interactions that involve trust, such as friendship and romantic relationships. Secondly, this is after all an experiment conducted in two laboratories; the investor and trustee were far apart, one in Texas and the other in California (1). I feel that the "social interaction" between subjects was unnatural. By removing the factors that would influence trust, the interaction may be reduced to a low-stakes gambling game, albeit one which the players have some control over their own winnings. Having said so, the players may be more willing to "trust" precisely because the stakes are low. Interestingly, the subjects were not asked to report their subjective experiences.

Zak et al (2003) offered some physiological evidence to suggest that interactions between players in the trust game resemble real-life interactions that involve trust. In their experiment, subjects were recruited to play a similar version of the trust game used by King-Casas et al; the differences were that investors started with $10 and each pair of subjects played only one round of the trust game (5). Immediately after a player decide whether or not to invest in (or repay) his or her partner, researchers took a blood sample from the player to test for 8 hormones that might be related to feelings of trust (5). Zak et al found that the level of oxytocin, which is both a hormone and neurotransmitter, was higher in trustees who received a greater sum of investment; the level of oxytocin correlates positively with the sum of investment received (5). Trustees with higher oxytocin levels reciprocated their investors' gesture with a greater sum of repayment, when compared to trustees' with lower oxytocin levels (5). Increased oxytocin levels were not found in investors, regardless the amount of money they transfer to their trustees (5). According to Zak et al, these findings are significant because oxytocin is associated with affiliative behaviors in animal studies (5). In humans, oxytocin is commonly associated with uterine contractions during delivery and lactation in female mammals (4). Oxytocin levels are also elevated during sex (6).

Could an elevated oxytocin level influence one's decision to trust others? As the trust game was played for only one round, it seems to me that the trustees would have to assume that their investors "trusted" them out of goodwill and did not transfer the money because they were taking a gamble or were greedy. Presuming that the trustees' opinion of their investors did not matter, could the release of oxytocin merely influence the trustees to act cooperatively and fairly because they have profited from that transaction, regardless whether they "trust" the investor? One may ask, how are we defining trust? According to Zak et al, we unconsciously trust others (5). That is, we may trust someone even if we have not consciously decided whether to do so. While I agree with this conception of trust, I am somewhat skeptical of the claim that oxytocin may be the "trust hormone." I am curious to find out whether the trustees in the study conducted by King-Casas et al would have higher levels of oxytocin after the trust game or if there would be any changes in their oxytocin levels from the early to later rounds of the game; unfortunately, the researchers did not draw blood from their subjects to test for oxytocin. It is also unclear whether trustees who played ten rounds (versus one) were consciously or unconsciously deciding to trust the investor for most part of the trust game.

From an evolutionary perspective, it seems probable for trust to be associated with reward circuits in the brain or even "feel-good" hormones such as oxytocin (6). Humans are social beings and our chances of survival and reproduction are enhanced if we worked together to protect or advance our shared interests. For us to be willing to rely on and cooperate with one another, we need to trust others. In this sense, the studies by King-Casas et al and Zak et al may help us to understand how the ability to trust may be encoded in our nervous systems. The issues of whether we consciously or unconsciously decide to trust others in most social interactions, or under what kinds of situations we may unconsciously trust others, may better help us to understand our own behavior. In my attempts to articulate what trust is, I have found a useful theoretical conception of trust for myself.

Soloman and Flores (2001) suggested that there are different forms of trust (7), and I find the distinction between simple trust and authentic trust to be helpful. According to these authors, simple trust "demands no reflection, no conscious choice, no scrutiny, no justification" and may be "viewed as a focused optimism" (7). In comparison, one exhibits authentic trust when one "is well aware of the risks, dangers, and liabilities of trust, but maintains the self-confidence to trust nevertheless" (7). It seems that when we unconsciously trust someone, we may be showing simple trust. This may possibly be related to physiological changes, such as increased oxytocin in our bodies in response to the benevolent actions of others. When we consciously decide to trust someone after having assessed whether he or she may be trustworthy, we may be showing authentic trust. This involves cognitive processes and may (or may not) be related to physiological changes. Authentic trust, I think, involves magnanimous faith; it may not necessarily be guided by an unconscious or conscious prediction of reward in one's interaction or relationship with others. Learning about the different forms of trust has helped me to make sense of my experiences involving trust.

Is it possible to understand trust from a neurological standpoint? I doubt anyone knows the answer although neuroscientists are searching for ways to study trust. While King-Casas et al (2005) have created a new procedure for studying brain activity during social interactions and their findings appear to be promising, one should keep in mind that these researchers are speculating about the role of the caudate nucleus in the trust building process. Unless scientists are able to implant some kind of device in the human brain that allows them to collect data on brain activity as subjects go about their daily lives, it seems almost improbable, I think, to fully understand trust from a neurological standpoint. With regards to whether trust may be rebuilt after betrayal, the findings of King-Casas et al (if their interpretations are valid) may suggest that the betrayed party is likely to trust again if over time, the traitor proves himself or herself to be trustworthy. The development of trust may be contingent on the consistency of positive reinforcements the betrayed party receives in future interactions with the traitor. Logically, whether trust may be rebuilt also depends on the nature of betrayal and severity of the consequences that result from the betrayal.

Presently, scientific research on the neurobiology of trust is at its infancy. More research is needed to answer the three questions I have explored in this paper. In the immediate future, there are several issues that may need to be addressed. Firstly, although the trust game is a useful paradigm, are researchers studying "trust" or a specific kind of trust that involves monetary rewards? Could the pattern of activity at the caudate nucleus be observed only when people play the trust game? Secondly, do player interactions in the trust game reliably mirror social interactions that involve trust? How would higher stakes influence the process of trust building? Thirdly, is there any relation between activity at the caudate nucleus and the release of oxytocin? Lastly, in order to understand trust and the process of building trust, it may be necessary to study the effects of betrayal on the human brain. How may the experience of a breach of trust influence activity in brain areas that are speculated to be the neural correlates of trust?

While we are all familiar with the experience of trust, it can be difficult to articulate what trust is. It is even more challenging for neuroscientists to study trust under laboratory conditions. This does not mean, however, that research on the neurobiology of trust is futile although it is important to recognize its limitations. While some people may share the concern that such research may be exploited and could result in dire social consequences (8), it is probable that the new knowledge we acquire about the neural correlates of trust and of the process of building trust would someday help individuals, who are unable or afraid to trust, build richer and more connected relationships with others.


1) Human Neuroimaging Lab , "Getting to Know You: Reputation and Trust in a Two-Person Economic Exchange"

2) Human Neuroimaging Lab , What is hyperscanning?

3) Neuropsychology, Medical Psychology, and Psychology Resources , Information about the caudate nucleus

4) Meyer, Jerrold, and Quenzer, Linda. Psychopharmacology: Drugs, the Brain, and Behavior. Sunderland, Massachusetts: Sinauer Associates, 2005.

5) The New York Times , "Looking into the Brains of the Stingy" by Virginia Postrel

6) The Financial Times , "A Trust Fund that Gives a Fillip to the Feel-Good Factor" by Alison Beard

7) Soloman, Robert, and Flores, Fernando. Building Trust in Business, Politics, Relationships, and Life. New York: Oxford University Press, 2001.

8) The Seattle Times , "Trust But Verify with a Brain Scan" by Rick Weiss

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