Measuring Trust in Social Neuroeconomics:
a Tutorial
Jan B. Engelmann
A brief outline of the eld of Neuroeconomics
Neuroeconomics is a relatively recent research area that is based on
the amalgamation of various disciplines, including experimental economics, experimental psychology and cognitive neuroscience. The
combination of methods from these fields has allowed researchers
to design experiments investigating brain function during decisionmaking. Recent experiments in neuroeconomics have significantly
furthered our understanding of the neural mechanisms involved in
economic and social decisions. On the one hand, recent advances in
neuroeconomics have lend support to the validity of economic models
by demonstrating that parameters central to economic models, such as
decision-utility (e.g. Knutson et al., 2005) and reinforcement learning
(Montague et al., 2004) are represented in the brain. On the other
hand, they have offered refinements of well-established views held by
traditional economics, such as the rational-agent model. Against the
assumption that economic behavior is purely rational, a wealth of converging evidence from behavioral and neuroimaging data underline
the role of emotions in decision-making in both financial (McClure
et al., 2004), as well as social (Sanfey et al., 2003) settings.
Since the current paper is directed at an audience outside of the
fields that constitute neuroeconomics, we briefly review some of the
core methodologies employed within neuroeconomics and related
disciplines to investigate the neural mechanisms of decision-making
before we discuss recent advances on the neural correlates of social
preferences using the example of trust.
Methodologies commonly employed in
Neuroeconomics
One fundamental building block of experiments in neuroeconomics
is the behavioral paradigm that is employed to investigate decisionmaking. Behavioral paradigms are typically taken from experimental
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DOI 10.51686/HBl.2010.1.17
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economics and psychology. Examples include the Trust Game (e.g.
Berg et al., 1995; Kosfeld et al., 2005), in which participants are faced
with the decision to entrust an anonymous person with their money, or the risky choice task (e.g. Engelmann and Tamir, 2009), in
which participants choose between lotteries with different levels of
risk and real financial consequences. Behavioral paradigms are then
adapted for use with methods from cognitive neuroscience, such as
non-invasive brain imaging and stimulation, which require specific
experimental design considerations related to timing and randomization of events in order to optimize statistical analyses. Common
neuroscience methods employed to study the brain in neuroeconomics include functional Magnetic Resonance Imaging (fMRI), which
allows researchers to observe brain function during decision-processes,
transcranial magnetic stimulation (TMS), through which a temporary
lesion in a targeted brain region can be generated, and pharmacological manipulations, which can temporarily alter the amount of a
targeted neurotransmitter within the central nervous system.
By far the most commonly used tool to investigate the neural basis
of economic and social decision-making is fMRI. This procedure
involves placing participants inside an MRI scanner, which provides
a record of neuronal activity in form of the Blood Oxygen LevelDependent (BOLD) response within the brain while participants
make decisions in the context of an experimental paradigm. The
BOLD response is reflective of the amount of oxygen that is being
delivered to neurons through the blood stream. Changes in blood
oxygenation levels lead to localized changes in the ferromagnetic
properties of blood that can be visualized by the MRI scanner.
Specifically, an increased BOLD signal in a certain brain region
is indicative of an increase in the amount of oxygen within the
blood stream, which in turn is reflective of an increased demand
for energy made by active neurons (e.g. Raichle and Gusnard, 2002).
Researchers can then couple the data provided by the fMRI scanner,
the BOLD signal, with behavioral data obtained from participants’
task performance that reflects economic and social preferences to
localize regions involved in producing behavior using statistical
analyses based on the General Linear Model. To accomplish this feat,
recent experiments have used model-based approaches to investigate
brain function (O’Doherty et al., 2007). In such sophisticated analyses, quantitative computational models estimate parameters reflective
of participants’ behavior that are relevant to the cognitive processes
underlying economic decision-making. These variables can then be
used to make predictions about what patterns of activation a given
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brain region should follow if it is involved in relevant computations
that lead to the observed behavior. Specifically, brain regions whose
neural signals show significant correlation with parameters derived
from computational models are likely involved in the computations
that produce the decisions of interest. Using such model-based fMRI
analyses, O’Doherty et al. (2003) have demonstrated that neurons
in the ventral striatum make specific predictions about the reward
contingencies of the environment in the form of the reward prediction error1 (O’Doherty et al., 2003), a finding that had previously
been demonstrated using invasive electrophysiological recordings in
monkeys (Schultz et al., 1998).
One drawback of fMRI is that it provides correlational information, meaning that brain regions identified to be involved in the
decision-process by this approach only correlate with the observed
behavior that was performed inside the scanner. While considerable
evidence indicates that regions identified via this method are involved in the computations that underlie behavior (Logothetis et al.,
2001), causal inferences in the form of brain region X causes behavior
Y can only be made in the presence of converging evidence. Such
converging evidence is commonly provided by studies from patients
with localized brain lesions, that, after removal of brain tissue, show
aberrant performance on decision-making tasks (e.g. Bechara et al.,
1996). It is, however, often difficult to come by patients with focal
damage to brain regions of interest and this approach suffers from
its own drawbacks. Recent technological advances have made noninvasive stimulation techniques that can be applied to healthy human participants, such as Transcranial Magnetic Stimulation (TMS)
and transcranial Direct Current Stimulation (tDCS) available to
researchers in neuroeconomics. Such brain stimulation approaches
can provide converging causal evidence that directly implicate brain
regions in decision-processes.
Transcranial Magnetic Stimulation (TMS) involves applying
high-intensity magnetic pulses via a magnetic coil that is strategically placed over the scalp to alter the excitability of neurons within
specific regions of cortex. The transient magnetic pulse penetrates
1
Human and animal studies have repeatedly demonstrated that midbrain dopamine
neurons process rewarding stimuli, such as food or money. Expectation modulates
activity within these regions: when a rewarding stimulus is unexpected, firing rates
increase; when it is expected, midbrain dopamine neurons do not change their firing
rates, and when an expected reward does not occur, a depression in firing rates is observed. Such neuronal activity patterns closely follow predictions from reinforcement
learning theory about how an agent learns environmental contingencies to maximize
rewards (for review see Montague et al., 2004).
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Jan B. Engelmann
through scalp and skull to generate an electrical field that can either
stimulate or disrupt neurons within targeted areas of cortex. The
specific effects of TMS on the excitability of cortical neurons depends on variations in stimulation parameters, including intensity,
frequency and repetition amount of the stimulation (Fitzgerald et al.,
2002). Using specific stimulation parameters, researchers can create
a localized, temporary and fully reversible brain lesion and study the
effects of disabling activity within a region of interest on behavior.
Furthermore, temporally extended effects on cortical excitability
can be achieved via repetitive TMS (rTMS), in which trains of pulses
are delivered to the same brain region for several minutes. This approach has yielded important insights about the role of dorsolateral
prefrontal cortex (DLPFC) in social decisions (Knoch et al., 2006).
Using low-frequency rTMS, Knoch et al. (2006) disrupted activity
within the right DLPFC, a region previously implicated in executive control and inhibition of prepotent responses. Participants then
played the Ultimatum Game, in which they were asked to either
accept or reject unfair offers made by another participant (see below for a more detailed outline of the Ultimatum Game). In the
context of this game, accepting an unfair offer can be interpreted
as following mostly selfish impulses that maximize payoffs, while
rejecting such an offer is reflective of punishing norm violators, a
decision that is costly to the subject. Knoch et al. showed that participants whose right DLPFC was disrupted accepted significantly
more unfair offers compared to placebo stimulation, indicating that
the neural computations performed in DLPFC are important for
controlling self-interest and punishing unfair offers.
Pharmacological manipulations involve administering a drug
challenge using compounds known to change the availability of
the targeted neurotransmitter within the central nervous system
and subsequently testing the effects of altered brain chemistry on
behaviors of interest. Neurotransmitter systems that are commonly
targeted are the dopamine system, for instance via systemic administeration of the dopamine agonist L-DOPA (e.g. Pleger et al., 2009;
Eisenegger et al., 2010), and the oxytocin system (e.g. (Kosfeld et
al., 2005; Baumgartner et al., 2008). These systems have repeatedly
been implicated in core decision processes, with dopaminergic brain
regions being involved in signaling the rewarding value of stimuli,
while oxytocin is involved in mediating social behaviors. A recent
investigation of the role of serotonin (5-HT) in social decisionmaking demonstrated that decreased serotonin levels within the
central nervous system increased rejection rates of unfair offers in
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the Ultimatum Game (Crockett et al., 2008).Together with a known
role of serotonin in impulsive aggression, these findings underline
the importance of emotion regulation in ultimatum bargaining.
Finally, recent investigations in neuroeconomics have begun to
examine the role of genetic predispositions in economic and social
decision-making (e.g. Kuhnen and Chiao, 2009; Eisenegger et al.,
2010), as well as aberrant decision-making in clinical populations,
including patients with depression, schizophrenia and anxiety disorders (e.g. Roiser et al., 2010).
Social Neuroeconomics investigates the neural
basis of social preferences
Traditional economics assumes that economic behavior is primarily
based on material self-interest, such that rational decision-makers
should maximize their own payoffs. Considerable evidence from a
multitude of experiments, however, argues against this simplifying
assumption (for review see Kahneman, 2003). Some of the most striking examples in support of this notion are found during strategic
interactions (e.g. Fehr and Gachter, 2002). Experiments using games
in which one player’s actions have a direct impact on the payoffs of
other participants, have repeatedly demonstrated that people exhibit clear «social preferences». This means that player’s decisions are
other-regarding, taking into account the well-being of other players
even though, in the context of strategic interactions, such choices are
costly and therefore not in agreement with their own self-interest.
To illustrate how social preferences can affect behavior, consider
for example results from the following oft-cited experiment employing the Ultimatum Game (Güth et al., 1982). Two anonymous
players interact in the Ultimatum Game, the «proposer» and the
«responder». The proposer is allocated a certain sum of money, say
10 monetary units (MU), with the task to split the money as he
wishes. If the responder accepts the split, both participants earn
their respective amounts. If, however, the responder rejects the split,
neither of the participants earns any money. Traditional economics would argue that the responder should accept any amount offered by the proposer that is greater than 0 MU, as this strategy
would maximize his payoff. However, in the experiment by Güth
and colleagues, responders tend to reject small offers of 2 MU, a
20:80 split in favor of the proposer, about half of the time. Such
findings indicate that responders are willing to pay 2 MU, and
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Jan B. Engelmann
sometimes even more, to punish the proposer for his unfair offer.
These results have been replicated in a multitude of experiments
using the Ultimatum Game and generalize to different experimental
settings (for reviews see Camerer, 2003; Fehr, 2009). Such behavior
is easily interpreted as an expression of social preferences that goes
against the notion that behavior is motivated purely by self-interest.
Because of their ubiquity, social preferences have been incorporated
as social utilities in formal models of social preferences by assigning
subjective values to other’s well-being (for review see Fehr, 2009).
Measuring trust
Similar results have been obtained using a related experimental setup, referred to as the Trust Game. In a typical version of the Trust
Game, originally introduced by Berg et al. (1995), two anonymous
players called the «investor» and «trustee» interact by sequentially
exchanging monetary amounts as follows: The investor is allocated
a certain amount of money by the experimenter, say 10 MU, and
is asked to send any amount from her endowment to the trustee.
Known to both participants, the amount transferred by the investor
is then tripled by the experimenter. The trustee’s role is to decide
how to share her endowment with the investor, that is, how much
money to send back. The amount sent by the investor is taken to
reflect trust-taking, as she voluntarily makes herself vulnerable by
placing her resources at the disposal of the trustee. Her motive for
doing so is that the social risk she took could increase her financial
wellbeing, if her trust is reciprocated. The amount returned by the
trustee, then, is a measure of prosociality and trustworthiness.
In the most commonly used version of the Trust Game, referred to
as one-shot games, players interact with a different individual on each
trial in order to avoid reputation building. In such settings, according
to predictions made by the self-interest assumption of traditional
economics, investors should keep all their money to themselves, because they cannot expect any return from a purely self-interested
individual. Trustees, on the other hand, should not transfer back any
money, because in a one-shot game no financial gain is obtained
from repaying the investor.The results reported by Berg et al. (1995),
however, paint a different picture. They find that investors in their
experiment sent more than half their initial endowment on average
and that about 95% of the investment was repaid by trustees. These
results have been replicated across a multitude of cultures (for review
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see Camerer, 2003), indicating that people generally trust and are
trustworthy. Together, these results underline the notion that social
preferences interact with self-interest in the production of behavior.
What does the Trust Game measure? The following criticism in regard to the external and ecological validity of the one-shot Trust
Game is commonly raised: Behavior in the Trust Game does not
reflect trust as understood by the general public or the social sciences. Trust is a multifaceted concept that is significantly richer than
the behavior measured in the Trust Game. For instance, it is a fundamental building block of most interpersonal relationships such as
friendships, marriages, and parent-child relations, but is also crucial
for economic transfers. Trust is also established and maintained by
social, legal and economic sanctions and can be furthered by communication. All these elements seem to be absent in the behaviors
measured by the Trust Game.
In one way, these criticisms are correct. In order to investigate trust
experimentally, researchers have to reduce this complex concept to
observables, that is, facets of behavior that can be measured quantitatively. This, however, is a desirable feature of the Trust Game, as it
provides a clean measure of trust that is free of confounding variables,
such as reputation-building, contractual pre-commitments and the
potential of punishment. Compared to face-to-face interactions,
which would be more naturalistic, anonymity controls for trustworthiness inferences from facial features that are typically made
within a fraction of a second (Todorov et al., 2009). Research has
demonstrated that such inferences are influenced to a great degree
by facial attractiveness (Stirrat and Perrett, 2010) and have little to
no predictive power about the actual trustworthiness of a person
(personal communication with Charles Efferson). Furthermore, repeated
interactions between two players would allow individuals to form
a reputation, which is driven by the individual’s self-interest to increase future payoffs. Therefore, experiments that ensure anonymity
between players and employ one-shot games provide the cleanest
measure of trust (Fehr, 2009).
Despite the above limitations of the experimental approach, recent
experiments in social neuroeconomics provide evidence in support
of the notion that the Trust Game captures relevant aspects of a
broader definition of trust. Indeed, considerable evidence underlines
the social nature of the decision-process involved in trust-taking in
the context of the Trust Game, which can be summarized as follows:
(1) behavioral and brain responses during trust-taking are distinct
from risk-taking and (2) trusting decisions in the context of Trust
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Games recruit neural circuitry commonly implicated in generating
models of other’s mental states, referred to as Theory of Mind (ToM).
Taken together, converging evidence from social neuroeconomics
indicates that participants interpret the Trust Game as a strategic
environment, underlining the ecological validity of the Trust Game.
The Neurobiology of Trust
1. Trust-taking is distinct from risk-taking. In the context of the Trust
Game, a decision to trust could inherently be a decision to take a
risk, be it a social risk. That is to say that an investor who decides to
transfer some or all of her endowment to the trustee forfeits a sure
payoff and risks losing some or all of it at the benefit of potentially
increasing her final payoff , depending on the decision of the trustee.
One question that arises from this notion is whether the brain makes
a distinction between trust-taking within the Trust Game and risktaking in a similarly framed choice setting. If trust-taking and risktaking produce differential behaviors and are processed by separate
brain systems, this would underline the social nature of trust-taking
in the context of the Trust Game. Two recent experiments using the
neuropeptide oxytocin (OT) have shed light on this question.
Oxytocin is a uniquely mammalian neuropeptide that is synthesized in the hypothalamus. It is released into the bloodstream via
the pituitary gland, where it acts as a hormone that is important for
partuition and lactation (Burbach et al., 2006). Of high relevance
to social neuroscience is the fact that the hypothalamus can also
release OT within the central nervous system, where it acts as a
neurotransmitter. OT binding sites are found throughout the brain,
but regions showing the highest OT receptor density are located
within the limbic brain and include the ventral striatum, nucleus
accumbens and amygdala (e.g. Hammock and Young, 2006).There is
now considerable evidence implicating OT in social behaviors and
cognition, including maternal behavior and pair bonding, social recognition, as well as social motivation and sexual behavior (for review
see Skuse and Gallagher, 2009). For instance, in rodents it has been
demonstrated that administration of an OT antagonist, which blocks
the action of OT in the central nervous system, decreases maternal
behavior (van Leengoed et al., 1987) while direct infusion of OT
into the brain leads to increased maternal behavior in animals known
to be non-maternal (Pedersen et al., 1982). Similarly, central infusion
of OT promotes pair bonding in monogamous voles (Williams et al.,
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1992), while a blockage of OT action within the brain decreases pair
bonding (Cho et al., 1999). In humans, it has repeatedly been demonstrated that intranasal administration of OT, which increases OT
concentrations within the brain (Born et al., 2002), facilitates social
cognition. A number of studies have used this approach, demonstrating that OT improves performance of difficult discriminations of
facial expressions in healthy humans (Domes et al., 2007a) as well
as in patients with autism spectrum disorder (ASD) (Guastella et al.,
2010), a neurodevelopmental disorder chararcterized by grave social
deficits (for review see Frith and Frith, 2003). Neuroimaging studies
have demonstrated that the social effects of OT are likely mediated
via the amygdala (Kirsch et al., 2005; Domes et al., 2007b), a region
that is important for social cognition and affective processing.
Kosfeld et al. (2005) conducted a version of the Trust Game with
two groups, one group that received intranasal administration of synthetic oxytocin and a control group that received inactive placebo
administration. Their results indicate that oxytocin administration
led to increases in trust-taking, as investors in the OT group had
significantly greater average transfers relative to the placebo control
group. To investigate whether oxytocin effects are specific to trusttaking in social settings or whether there is a more generalized effect
on risk-taking, the study included a risk task, in which participants
faced the same choices, except that they knew that the amount of
money returned was determined by a random mechanism implemented by a computer instead of a human counterpart. Interestingly,
despite the fact that the choice context was the same, oxytocin had
no effect on average transfer rates in this condition. Finally, the effect
of oxytocin could be to increase prosocial inclinations in general,
in which case, increased transfer rates should not only be observed
for investors, but also for trustees. No effect of oxytocin for trustees
was observed, indicating that the effect of oxytocin was specific for
trust-taking. Taken together, results from Kosfeld et al. (2005) indicate that oxytocin increases trust-taking, and that these effects are
specific and do not generalize to risk-taking, nor lead to increased
prosocial behaviors. Because of oxytocin’s known role in facilitating
social behaviors, the authors interpreted these results as indicating
that OT’s effects on trust-taking are mediated via reducing social
anxiety, such as betrayal aversion (Bohnet and Zeckhauser, 2004).
A more recent neuroimaging investigation by Baumgartner et al.
(2008) provides supporting evidence for this conclusion. Similar to
the experiment by Kosfeld et al. (2005), this experiment included
two groups of participants, one that received oxytocin and a placebo
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control group.While undergoing fMRI, participants played both the
Trust Game and a risk game in which choices are equivalent to
the Trust Game, except that reinforcement was based on a random
algorithm implemented by a computer instead of choices made by
another player. Importantly, about halfway through the experiment
participants were informed that their decisions to trust and take risks
were not returned on half the trials. Behavioral results indicate that,
while group differences before feedback were not statistically significant, behavioral adaptation to feedback was significantly affected
by oxytocin. Specifically, average transfers increased after feedback
in the OT group and decreased significantly in the placebo group.
No such differences were observed in the risk game, indicating
that these effects are specific to social risks taken in the Trust Game.
These results are consistent with the notion put forward by Kosfeld
et al. (2005) that OT reduces fear of social betrayal. Neuroimaging
findings from Baumgartner et al. (2008) lend further support to this
notion. They demonstrate an increase in activity in the amygdala
during trust-taking in the postfeedback phase relative to prefeedback
in the placebo group, but no such effect in the OT group.The effect
of OT was therefore to decrease activity in the amygdala, which was
associated with increased trust-taking even after participants were
informed of betrayal. Taken together with an extensive literature
implicating the amygdala in fear processing and emotional relevance
detection (e.g. Phelps and LeDoux, 2005), as well as studies showing that OT decreases fear responses by modulating activity in the
amygdala (Kirsch et al., 2005; Domes et al., 2007b), these findings
are consistent with the notion that OT reduces social fear by decreasing reactivity of the amygdala. Since such effects are specific to
social tasks, OT administration likely leads to a reduction of social
anxiety, such as betrayal aversion (Bohnet and Zeckhauser, 2004).
2. Trust-taking recruits neural circuitry involved in perspective-taking. The
differential effects of OT on behavior in risk and Trust Games indicate that there is a specifically social aspect to trust-taking that is
mediated via the social neuropeptide OT. A further factor that plays
an important role in game theoretical paradigms, such as the Trust
Game, is the ability to reason about other people’s mental states, also
termed Theory of Mind (ToM). When interacting with other human
players in the context of economic games, knowledge of another
player’s state of mind, that is his level of fairness, current emotional
state and how he might react to my behavior, can help predict the
opponent’s future actions and provide a competitive advantage. Even
in one-shot games, in which such knowledge cannot be obtained,
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strategic thoughts concerning the perspective of an opponent have
been shown to occur (Costa-Gomez et al., 2001). Such strategies
are absent when outcomes of decisions are determined by random
processes, such as in the context of the risk game, or when playing
against a computerized opponent.
Social perspective taking has been extensively studied by previous
research. Developmental studies using the false-belief task in children have demonstrated that the ability to infer another’s perspective
does not fully develop until the age of 5 (Baron-Cohen et al., 1985)
and is impaired in patients with autism spectrum disorder (ASD) (e.g.
(Baron-Cohen et al., 1994). Neuroimaging studies employing such
tasks have implicated a specific network of brain regions in the capacity to infer other’s mental states, including the medial prefrontal
cortex, superior temporal sulcus and temporoparietal junction (for
reviews see Amodio and Frith, 2006; Hein and Knight, 2008).
Supporting evidence for the notion that the Trust Game does
indeed measure social aspects of trust would be provided by results
from behavioral and neuroimaging experiments underlining the notion that investors engage in perspective-taking. In one of the first
neuroimgaing investigations of trust-taking by McCabe et al. (2001),
participants played a number of games that included the Trust Game
outlined above. On half of the trials, participants interacted with
another human player, while on the other half, the opponent participants faced was a computer. Results from this study indicate that
participants could be distinguished based on two distinct strategies
they employed to solving this task: (1) cooperators that considered
the well-being of other players and (2) non-cooperators. Interestingly,
these different strategies led to differential brain activation patterns.
Only cooperators showed significant activation in mPFC, specifically
the anterior paracingulate cortex, when interacting with a human
compared to a computer counterpart. Given the known involvement
of mPFC in mentalizing (Amodio and Frith, 2006), the authors concluded that generating a model of the other player’s mental state plays
an important role in producing cooperative behavior.
A number of related studies lend support to this notion. A
Positron Emission Tomography (PET)2 study by Gallagher et al.
(2002) showed activation in anterior paracingulate cortex when
participants played a game of «stone-paper-scissors» against a human
compared to a computer opponent. Similarly, Rilling et al. (2004)
2
Positron Emission Tomography is a neuroimaging method that uses radioactive
tracers injected into the blood stream. Using specific tracers such as fludeoxyglucose,
this method can provide images reflective of the energy consumption in the brain.
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demonstrated activations in anterior paracingulate cortex, as well
as superior temporal sulcus and posterior cingulate cortex when
participants played the Ultimatum Game or the related Prisoners
Dilemma Game against a human compared to a computer opponent. Together, these results demonstrate that fundamentally
different choice strategies are employed when participants interact
with human compared to non-human partners in various gametheoretical settings. Neurobiological evidence showing that central
nodes within the Theory of Mind network are intimately involved
in decision-making when participants interact with other human
players lends support to the notion that social choice involves the
generation of models representing another agent’s mental state.
In repeated Trust Games, the same participants interact with each
other over several rounds of the Trust Game by sequentially placing
both participants in the roles of the trustee and the investor. This
approach allows participants to form a reputation and thus increases
the ecological validity of the experiment. Forming a reputation is
similar to building a mental model of the trustworthiness of the
other player that likely involves mentalizing. Various experiments
have employed this approach in combination with hyperfunctional
fMRI, which provides a simultaneous record of neuronal activity
from multiple brains. A recent study by King-Casas et al. (2005)
showed that activity within the trustee’s caudate nucleus encodes
her intention to trust on the next trial. Specifically, during trials
immediately before trustees decided to increase trust, signal in the
caudate nucleus increased after the investor’s decision was revealed.
Importantly, as participants formed a model of the other player, this
response shifted forward in time, such that in later rounds of the
Trust Game, an increased peak predicting the intention to trust on
the next trial was observed even before the investor’s decision was
revealed. A similar temporal shift was observed in cross- and withinbrain correlations between activity in the caudate nucleus, as well
as mid and anterior cingulate cortex, a region that is adjacent to
the anterior paracingulate cortex. Together, these results indicate
that caudate responses transitioned from reactive to anticipatory as
a mental model of the other player was formed in a fashion that
resembles the reward prediction error. Corroborating evidence for
this notion was provided by a separate behavioral study, in which
the authors showed that trustees predictions about the magnitude
of investor’s backtransfers significantly improved after playing about
eight rounds of the Trust Game with the same partner. These results indicate that trustees build a mental model of their partner’s
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trustworthiness, whose predictions become more accurate with
increasing experience. Neurobiological evidence implicates a brain
network involving caudate nucleus and anterior cingulate cortex in
mediating this process.
A follow-up study using the same experimental setup demonstrated that specialized regions within cingulate cortex encode decisions of others and oneself (Tomlin et al., 2006), with regions in
mid-cingulate most responsive to the subject’s own decision, while
anterior and posterior regions represented the other subject’s choice.
Importantly, in non-social control experiments that kept the visual
and motor requirement constant, no such activations were observed
within cingulate cortex. Together, these findings demonstrate that
the cingulate cortex performs social agency computations to perform distinctions between actions made by the self and others, implicating this region in generating mental models of other players.
Finally, Krueger et al. (2007), using a multiround Trust Game,
demonstrated activation of the paracingulate cortex during trials in
which participants decided to trust. An interesting feature of this
study was the observation that two types of relationships developed
in the course of repeated interactions between two players: (a) in
nondefector relationships trustees never defected on investors’ decisions to trust while (b) in defector relationships players experienced
several breaches of trust throughout the experiment. Interestingly,
activity in the paracingulate region differentiated between such
relationships, showing significantly greater activation during trustbuilding in non-defectors compared to defectors. These results
indicate that an engagement of the mentalizing network during the
relationship-building phase of the experiment led to greater levels
of cooperation.
Taken together, results reviewed above lend support to the notion
that participants interacting with human opponents recruit central
nodes of the Theory of Mind network, such as STS and mPFC, while
such activations are absent when interacting with non-intentional
entities, such as a computer. Evidence from multiround Trust Games
indicates that such activation is involved in generating models of
other player’s mental states. To return to our original question, these
results therefore lend further support to the notion that trust-taking
in the context of the Trust Game is perceived as a social act that
involves generating models of other players’ mental states and their
social preferences, even in the context of one-shot games.
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Conclusions
Social interactions are inherently complex and can be influenced
by a multitude of factors. In order to be able to experimentally
investigate such complex interactions, the experimental approach
employed in social neuroeconomics requires a reduction of complicated concepts, such as trust, to observable facets of behavior
that can be quantitatively measured. On the one hand, this leads to
experimental tasks that measure very specific behaviors that, at first
glance, do not appear naturalistic nor easily generalize to real-world
interpersonal interactions. On the other hand, the experimental approach allows for observation of quantifiable behaviors that can be
coupled with methods from neuroscience to enable investigations
of brain function. Importantly, via the example of the Trust Game
we have demonstrated that, despite the reductionist experimental
approach employed in neuroeconomics, the main features of the
behavior of interest are preserved. Specifically, participants interpret
the Trust Game as a social environment that requires strategic interactions with another player that are distinct from simple risk-taking
and involve generating models of other player’s social preferences.
Removal of features that we cannot directly measure but might
systematically influence our results is necessary to obtain clearly
interpretable data free from confounding factors. While this leads to
limited insights achieved by any given experiment, there is always
the possibility to perform follow-up experiments that investigate
additional features of the behavior of interest.
I would to thank Grit Hein, Christian Ruff and Friederike Meyer for
comments on earlier versions of this manuscript.
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— Dr. Jan Engelmann arbeitet im Rahmen des Projekts «Vertrauen verstehen» an
einer Habilitation mit dem Thema «Die emotionalen und neurobiologischen Ursachen
von Vertrauen».
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