Tryptophan promotes interpersonal trust.

Short Report " Every kind of peaceful cooperation among men is primarily based on mutual trust and only secondarily on institutions such as courts of justice and police, " Albert Einstein (1950) once said. Indeed, interpersonal trust is an essential element of social life in general and an important determinant of cooperative behavior in particular (Pruitt & Kimmel, 1977; Yamagishi, 1986). After all, most people will cooperate only if they expect others to do so as well, which makes mutual trust an important precondition for establishing mutual cooperation. Pharmacological studies in rats and humans suggest that the neurotransmitter serotonin (5-HT) plays a crucial role in promoting cooperative behavior (Crockett, 2009), which can be enhanced by increasing the 5-HT level through administration of selective serotonin reuptake inhibitors (Knutson et al., 1998; Tse & Bond, 2002) and can be reduced by lowering the 5-HT level through tryp-experiment reported here, for the first time, we focused on the link between 5-HT and the key precursor to cooperation: interpersonal trust. We tested whether mutual trust can be promoted by administering the food supplement L-tryptophan (TRP), the biochemical precursor of 5-HT. TRP is an essential amino acid contained in food such as fish, soybeans, eggs, and spinach. TRP supplementation is known to increase plasma TRP levels and to influence brain 5-HT synthesis (Markus, Firk, Gerhardt, Kloek, & Smolders, 2008). We expected to find an effect of TRP on interper-sonal trust because the medial prefrontal cortex, the brain region associated with trust-related decisions receives serotonergic projections from neurons in the raphe nuclei, the principal source of 5-HT release in the brain. It is thus plausible, if not likely, that the activation of the medial prefrontal cortex is modulated through serotonergic projections— which we aimed to target by supplementation with TRP. Method We investigated the link between TRP supplementation and interpersonal trust in 40 healthy adults exposed to an oral dose of either TRP or a neutral placebo (n = 20 in each condition; for details on the method, see the Supplemental Material available online). Participants arrived at the laboratory in pairs, took their dose of TRP or placebo, and about an hour later performed an unrelated joint-action task. We then measured interpersonal trust by having each pair perform the Trust Game (Camerer & Weigelt, 1988)—a task that measures the extent to which one person (the truster) trusts another person (the trustee), as indicated by money units …