Oxytocin and Trust: The Neuroeconomics Evidence

Since the landmark 2005 study by Kosfeld and colleagues demonstrating that intranasal oxytocin increases trust in an economic game, the relationship between this neuropeptide and human trust has become one of the most intensively studied topics in neuroeconomics. This page focuses specifically on the experimental economics evidence – trust games, ultimatum games, investment paradigms, and neuroimaging studies – that has shaped our understanding of how oxytocin modulates economic decision-making, risk tolerance, and betrayal aversion. For a broader overview of oxytocin’s role in trust and social behaviour, see our general trust page.

The Trust Game: An Economist’s Tool for Measuring Trust

How the Trust Game Works

The trust game, originally devised by Berg, Dickhaut, and McCabe (1995), is a two-player sequential exchange game. Player 1 (the “investor” or “trustor”) receives an endowment of money and can transfer any portion of it to Player 2 (the “trustee”). The transferred amount is multiplied (typically tripled) by the experimenter before reaching Player 2, who then decides how much to return to Player 1. The investor’s transfer amount is used as a behavioural measure of trust, since larger transfers indicate greater willingness to accept the vulnerability inherent in trusting another person with one’s resources.

What makes this paradigm so powerful for neuroeconomics research is that it operationalises trust as a measurable, quantifiable behaviour with real financial consequences. Unlike questionnaire measures of generalised trust, the trust game captures actual decision-making under uncertainty – making it an ideal tool for studying how pharmacological manipulations, including oxytocin administration, alter trust behaviour.

The Kosfeld Breakthrough: Oxytocin Increases Trust (2005)

Design and Findings

The study that launched the neuroeconomics of oxytocin was published by Kosfeld, Heinrichs, Zak, Fischbacher, and Fehr (2005) in Nature. In a double-blind, placebo-controlled, between-subjects design, 58 healthy male participants received either 24 IU intranasal oxytocin or placebo 50 minutes before playing a trust game with real monetary stakes (up to 12 Swiss francs per round).

The results were striking: investors who received oxytocin transferred significantly more money to their trustees than those who received placebo. Specifically, 45% of oxytocin-treated investors transferred the maximum possible amount, compared with only 21% of placebo-treated investors. The mean transfer was approximately 17% higher in the oxytocin group. Crucially, the authors included a risk control condition in which investors could make the same transfers to a computerised lottery with identical expected payoffs – and oxytocin had no effect on transfers in this condition. This dissociation demonstrated that oxytocin specifically increased social trust (willingness to accept vulnerability to another person), not general risk tolerance (Kosfeld et al., 2005).

Interpretive Debates

The Kosfeld study generated immediate debate about the mechanism underlying oxytocin’s trust-enhancing effect. Was oxytocin reducing social anxiety? Increasing social approach motivation? Altering the computation of expected betrayal? Or simply making other people seem more attractive or trustworthy? Subsequent studies have attempted to disentangle these possibilities, and the picture that has emerged is more nuanced than the initial “trust hormone” narrative suggested.

Betrayal Aversion: The Baumgartner Study (2008)

Neural Mechanisms of Oxytocin’s Trust Effect

Baumgartner, Heinrichs, Vonlanthen, Fischbacher, and Fehr (2008) advanced the Kosfeld paradigm by adding functional magnetic resonance imaging (fMRI). Participants played a trust game, received feedback indicating that their trust had been betrayed (the trustee had returned nothing or very little), and then played additional rounds. Under placebo, participants who experienced betrayal dramatically reduced their subsequent trust – a rational Bayesian updating response. Under oxytocin, however, participants continued to trust at high levels even after betrayal feedback.

The neuroimaging data revealed that oxytocin reduced activation in the amygdala and the caudate nucleus – regions involved in fear processing and the prediction of social outcomes, respectively – in response to betrayal signals. The authors interpreted this as evidence that oxytocin does not make people naively trusting in all contexts, but rather specifically attenuates the neural alarm signals that normally trigger betrayal aversion and protective withdrawal after negative social feedback (Baumgartner et al., 2008).

Clinical and Ethical Implications

The finding that oxytocin could maintain trust even after betrayal raised immediate ethical concerns. If oxytocin reduces the adaptive response to untrustworthiness, could it be exploited in commercial or political contexts to induce maladaptive trust? Zak (2008) discussed these implications, noting that while laboratory effects are modest and context-dependent, the principle that a simple nasal spray could alter economic decision-making in favour of the person administering it has genuine societal implications.

Dose-Response Relationships and Replication Challenges

The Replication Landscape

The oxytocin-trust literature has not been without controversy. Several replication attempts have yielded mixed results. Nave, Camerer, and McCullough (2015) conducted a comprehensive meta-analysis of intranasal oxytocin effects on trust and concluded that the overall effect size was small and that publication bias may have inflated early estimates. Their analysis raised important methodological questions about sample sizes, dosing protocols, and the assumption that intranasal oxytocin reliably reaches behaviourally relevant brain concentrations.

However, Declerck, Boone, and Kiyonari (2010) demonstrated that oxytocin’s effect on trust is strongly moderated by social context. In their study, oxytocin increased trust only when participants had been primed with positive social cues – suggesting that the peptide amplifies pre-existing social signals rather than uniformly increasing trust. This interaction effect helps explain why direct replications with different social contexts or populations sometimes fail to reproduce the original Kosfeld finding.

Individual Differences: Genetics and Personality

The trust-enhancing effect of exogenous oxytocin varies substantially across individuals. Keri and Kiss (2011) showed that polymorphisms in the OXTR gene (particularly the rs53576 SNP) moderate the effect of intranasal oxytocin on trust behaviour. GG homozygotes – who are independently associated with higher trait empathy and social sensitivity – showed larger trust increases following oxytocin administration than A allele carriers. This gene-by-drug interaction suggests that the oxytocinergic system’s capacity to modulate trust is partly determined by the density and function of endogenous oxytocin receptors.

Beyond the Trust Game: Other Neuroeconomic Paradigms

Ultimatum Game and Generosity

The ultimatum game – in which a proposer offers a split of money and a responder can accept or reject the offer (rejection means both players get nothing) – has also been used to study oxytocin’s effects on economic decision-making. Zak, Stanton, and Ahmadi (2007) found that intranasal oxytocin increased generosity in the proposer role by 80% compared to placebo. Interestingly, oxytocin did not affect rejection thresholds in the responder role, suggesting that its primary effect is on prosocial giving rather than fairness enforcement.

Public Goods Games and Cooperation

De Dreu, Greer, Van Kleef, Shalvi, and Handgraaf (2011) extended the neuroeconomics of oxytocin to inter-group contexts using a public goods game with competing groups. Their findings introduced an important nuance: oxytocin increased cooperation within the in-group but did not increase (and in some conditions decreased) cooperation with out-group members. This “parochial altruism” effect challenged the simplistic “prosocial hormone” narrative and suggested that oxytocin promotes group-preferential, rather than universal, cooperation – a finding with implications for the understanding of oxytocin’s darker social effects.

Dictator Games and Pure Altruism

In dictator games – where one player simply allocates resources with no possibility of rejection – oxytocin effects have been inconsistent. Barraza and Zak (2009) found that endogenous oxytocin levels (measured via blood draw) correlated positively with generosity in a dictator game after watching an emotional video. However, Weisman, Zagoory-Sharon, and Feldman (2014) found that intranasal oxytocin’s effect on dictator game giving depended on the social relationship context – oxytocin increased giving to in-group members but not to strangers or out-group members, consistent with the parochial cooperation framework.

Neuroimaging Evidence: How Oxytocin Alters Economic Decision Circuits

Amygdala Modulation

Across multiple neuroimaging studies, the most consistent neural effect of intranasal oxytocin during economic decision-making is reduced amygdala activation in response to social threat and uncertainty cues. Kirsch et al. (2005) demonstrated that oxytocin reduced amygdala responses to threatening faces, and Baumgartner et al. (2008) extended this to economic betrayal signals. The amygdala is a critical hub for computing social threat and triggering defensive responses – and its modulation by oxytocin may be the proximal neural mechanism through which the peptide facilitates trust by dampening the fear of being exploited.

Ventral Striatum and Reward Processing

Rilling et al. (2012) used fMRI to examine how oxytocin affected neural responses during reciprocated and unreciprocated cooperation in an iterated prisoner’s dilemma. Oxytocin increased ventral striatum activation during reciprocated cooperation – suggesting that the peptide enhances the subjective reward value of mutual trust. It also increased activation in the anterior insula and anterior cingulate cortex during unreciprocated cooperation, which the authors interpreted as enhanced empathic concern when trust is violated, rather than reduced awareness of betrayal.

Prefrontal Regulation

Domes et al. (2007) demonstrated that oxytocin enhanced the ability of participants to infer the emotional states of others from eye region photographs (the “Reading the Mind in the Eyes” test). This suggests that oxytocin may facilitate trust partly by improving the accuracy of social cognition – enabling investors to better assess the likely trustworthiness of their counterparts. The medial prefrontal cortex, which is involved in mentalising and theory-of-mind computations, shows enhanced connectivity with the amygdala under oxytocin – a pattern consistent with improved integration of social information into economic decisions (Sripada et al., 2013).

The “Dark Side” of Oxytocin in Economic Contexts

Gullibility, Exploitation, and Intergroup Bias

The finding that oxytocin maintains trust after betrayal (Baumgartner et al., 2008) raises the concern that oxytocin could make individuals more gullible or exploitable. Mikolajczak et al. (2010) directly tested this using a trust game with explicit cues about trustee reliability. They found that oxytocin increased trust toward trustees described as reliable but did not increase trust toward trustees described as unreliable – suggesting that the peptide enhances trust selectively, using available social information, rather than inducing blanket gullibility.

Nonetheless, De Dreu et al. (2010) demonstrated that oxytocin can amplify ethnocentric bias in economic games, promoting defensive aggression and pre-emptive strikes against out-group members when intergroup competition is salient. This suggests that oxytocin’s role in economic decision-making cannot be separated from the broader social context – including group identity, threat perception, and cultural framing.

Methodological Considerations and Future Directions

The neuroeconomics of oxytocin faces several methodological challenges. The reliability of intranasal oxytocin delivery to brain target regions remains debated (Leng & Ludwig, 2016). Sample sizes in many early studies were small by current standards, raising concerns about statistical power and false-positive rates. The exclusive use of male participants in several foundational studies (including Kosfeld et al., 2005) limits generalisability, as sex differences in oxytocinergic system function are well documented (Rilling et al., 2014).

Future research will benefit from larger, pre-registered trials; inclusion of both sexes; exploration of dose-response relationships; and integration of genetic, epigenetic, and endogenous oxytocin measures alongside exogenous administration paradigms. The development of selective oxytocin receptor agonists with improved brain penetrance may also help resolve debates about whether intranasal oxytocin’s behavioural effects are truly mediated by central receptor binding.

Frequently Asked Questions

References

1. Barraza, J.A. & Zak, P.J. (2009). Empathy toward strangers triggers oxytocin release and subsequent generosity. Annals of the New York Academy of Sciences, 1167, 182–189.
2. Baumgartner, T. et al. (2008). Oxytocin shapes the neural circuitry of trust and trust adaptation in humans. Neuron, 58(4), 639–650.
3. Berg, J., Dickhaut, J. & McCabe, K. (1995). Trust, reciprocity, and social history. Games and Economic Behavior, 10(1), 122–142.
4. De Dreu, C.K.W. et al. (2010). The neuropeptide oxytocin regulates parochial altruism in intergroup conflict among humans. Science, 328(5984), 1408–1411.
5. De Dreu, C.K.W. et al. (2011). Oxytocin promotes human ethnocentrism. Proceedings of the National Academy of Sciences, 108(4), 1262–1266.
6. Declerck, C.H., Boone, C. & Kiyonari, T. (2010). Oxytocin and cooperation under conditions of uncertainty. Hormones and Behavior, 57(3), 368–374.
7. Domes, G. et al. (2007). Oxytocin improves “mind-reading” in humans. Biological Psychiatry, 61(6), 731–733.
8. Keri, S. & Kiss, I. (2011). Oxytocin response in a trust game and habituation of arousal. Physiology & Behavior, 102(2), 221–224.
9. Kirsch, P. et al. (2005). Oxytocin modulates neural circuitry for social cognition and fear in humans. Journal of Neuroscience, 25(49), 11489–11493.
10. Kosfeld, M. et al. (2005). Oxytocin increases trust in humans. Nature, 435(7042), 673–676.
11. Leng, G. & Ludwig, M. (2016). Intranasal oxytocin: myths and delusions. Biological Psychiatry, 79(3), 243–250.
12. Mikolajczak, M. et al. (2010). Oxytocin makes people trusting, not gullible. Psychological Science, 21(8), 1072–1074.
13. Nave, G., Camerer, C. & McCullough, M. (2015). Does oxytocin increase trust in humans? A critical review of research. Perspectives on Psychological Science, 10(6), 772–789.
14. Rilling, J.K. et al. (2012). Effects of intranasal oxytocin and vasopressin on cooperative behavior and associated brain activity in men. Psychoneuroendocrinology, 37(4), 447–461.
15. Rilling, J.K. et al. (2014). Sex differences in the neural and behavioral response to intranasal oxytocin and vasopressin during human social interaction. Psychoneuroendocrinology, 39, 237–248.
16. Sripada, C.S. et al. (2013). Oxytocin enhances resting-state connectivity between amygdala and medial frontal cortex. International Journal of Neuropsychopharmacology, 16(2), 255–260.
17. Weisman, O., Zagoory-Sharon, O. & Feldman, R. (2014). Oxytocin administration, salivary testosterone, and father–infant social behavior. Progress in Neuro-Psychopharmacology & Biological Psychiatry, 49, 47–52.
18. Zak, P.J. (2008). The neurobiology of trust. Scientific American, 298(6), 88–95.
19. Zak, P.J., Stanton, A.A. & Ahmadi, S. (2007). Oxytocin increases generosity in humans. PLoS ONE, 2(11), e1128.