Psych Discussion: Trust pervades human societies

Psych Discussion: Trust pervades human societies

Psych Discussion: Trust pervades human societies

Oxytocin increases trust in humans Michael Kosfeld1*, Markus Heinrichs2*, Paul J. Zak3, Urs Fischbacher1 & Ernst Fehr1,4

Trust pervades human societies1,2. Trust is indispensable in friend- ship, love, families and organizations, and plays a key role in economic exchange and politics3. In the absence of trust among trading partners, market transactions break down. In the absence of trust in a country’s institutions and leaders, political legitimacy breaks down. Much recent evidence indicates that trust contrib- utes to economic, political and social success

4,5. Little is known, however, about the biological basis of trust among humans. Here we show that intranasal administration of oxytocin, a neuro- peptide that plays a key role in social attachment and affiliation in non-human mammals6–8, causes a substantial increase in trust among humans, thereby greatly increasing the benefits from social interactions. We also show that the effect of oxytocin on trust is not due to a general increase in the readiness to bear risks. On the contrary, oxytocin specifically affects an individual’s willingness to accept social risks arising through interpersonal interactions. These results concur with animal research suggesting an essential role for oxytocin as a biological basis of prosocial approach behaviour.

In non-human mammals, the neuropeptide oxytocin has a central role in general behavioural regulation, particularly in positive social interactions. Aside from its well-known physiological functions in milk letdown and during labour, oxytocin receptors are distributed in various brain regions associated with behaviour9,10, including pair bonding, maternal care, sexual behaviour, and the ability to form normal social attachments 6–8,11–15.

Thus, oxytocin seems to permit animals to overcome their natural avoidance of proximity and thereby facilitates approach behaviour. Given that oxytocin is believed to promote social attachment and affiliation in non- human mammals, we hypothesized that oxytocin might also pro- mote prosocial approach behaviours—such as trust—in humans. Recent research has shown that neuropeptides cross the blood-brain barrier after intranasal administration16, providing a useful method for studying the central nervous system effects of oxytocin in humans17,18. We used a double-blind study design to compare trusting behaviour in a group of subjects that received a single dose of intranasal oxytocin with that of subjects in a control group that received placebo.

We analysed the effect of exogenously administered oxytocin on individuals’ decisions in a trust game with real monetary stakes19–22. In this trust game, two subjects interacting anonymously play either the role of an investor or a trustee (Fig. 1). First, the investor has the option of choosing a costly trusting action by giving money to the trustee. If the investor transfers money, the total amount available for distribution between the two players increases but, initially, the trustee reaps the whole increase. The trustee is then informed about the investor’s transfer and can honour the investor’s trust by sharing the monetary increase generated by the investor’s transfer. Thus, if the investor gives money to the trustee and the latter shares the proceeds of the transfer, both players end up with a higher

monetary payoff. However, the trustee also has the option of violating the investor’s trust. As sharing the proceeds is costly for the trustee, a selfish trustee will never honour the investor’s trust because the investor and the trustee interact only once during the experiment.

The investor is therefore caught in a dilemma: if he trusts and the trustee shares, the investor increases his payoff, but he is also subject to the risk that the trustee will abuse this trust. In the latter case, the investor is worse off than if he had not trusted at all and, adding insult to injury, the trustee has an unfair payoff advantage relative to the investor. Substantial evidence exists to show that humans are averse to such risks22–24. Moreover, the aversion of investors to abuse of trust seems to have an important role across different human cultures and social groups in the context of our game22,25. The investors have to overcome their aversion against these risks in order to trust, allowing us to address the question of whether oxytocin modulates this trusting behaviour in humans.

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Psych Discussion: Trust pervades human societies

LETTERS

Figure 1 | The trust game. Both subjects receive an initial endowment of 12 monetary units (MU). The investor can send 0, 4, 8 or 12 MU to the trustee. The experimenter triples each MU the investor transfers. After the investor’s decision is made, the trustee is informed about the investor’s transfer. Then the trustee has the option of sending any amount between zero and his total amount available back to the investor. For example, if the investor has sent 12 MU, the trustee possesses 48 MU (12 MU own endowment þ 36 MU tripled transfer) and can, therefore choose any back transfer from 0 to 48 MUs. The experimenter does not triple the back transfer.

The investor’s final payoff corresponds to the initial endowment minus the transfer to the trustee, plus the back transfer from the trustee. The trustee’s final payoff is given by his initial endowment plus the tripled transfer of the investor, minus the back transfer to the investor. At the end of the experiment, the earned MU are exchanged into real money according to a publicly announced exchange rate (see Methods). Each subject made four decisions in the same player role while paired with four different, randomly selected interaction partners.

1University of Zurich, Institute for Empirical Research in Economics, Blumlisalpstrasse 10, CH-8006 Zurich, Switzerland. 2University of Zurich, Department of Clinical Psychology and Psychotherapy, Zurichbergstrasse 43, CH-8044 Zurich, Switzerland. 3Center for Neuroeconomics Studies, Claremont Graduate University, Claremont, California 91711-6165, USA. 4Collegium Helveticum, Schmelzbergstrasse 25, CH-8092 Zurich, Switzerland. *These authors contributed equally to this work.

Our hypothesis that oxytocin increases the trusting behaviour of investors implies that the investors in the oxytocin group (n ¼ 29) will show higher money transfers than those in the placebo group (n ¼ 29). In fact, our data show that oxytocin increases investors’ trust considerably. Out of the 29 subjects, 13 (45%) in the oxytocin group showed the maximal trust level, whereas only 6 of the 29 subjects (21%) in the placebo group showed maximal trust (Fig. 2a). In contrast, only 21% of the subjects in the oxytocin group had a trust level below 8 monetary units (MU), but 45% of the subjects in the control group showed such low levels of trust. These differences in the distribution of trust result in higher average and median trust levels for subjects given oxytocin (Table 1). The investors’ average transfer is 17% higher in the oxytocin group (Mann-Whitney U-test; z ¼ 21.897, P ¼ 0.029, one-sided), and the median transfer in the oxytocin group is 10 MU, compared to a median of only 8 MU for subjects in the placebo group.