Trust pervades human societies^{1, 2}. Trust is indispensable in friendship, 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 contributes 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 neuropeptide that plays a key role in social attachment and affiliation in non-human mammals^{6, 7, 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.

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

Correspondence to: Markus Heinrichs^2,5Ernst Fehr^1,4 Correspondence and requests for materials should be addressed to E.F. (Email: efehr@iew.unizh.ch) and M.H. (Email: m.heinrichs@psychologie.unizh.ch).

Received 20 April 2005; Accepted 5 May 2005