Led by Lorenza Colzato, a psychologist from the University of Leiden in the Netherlands, the researchers investigated whether tryptophan -- an amino acid found in red meat, dairy products, nuts, seeds, turkey, tuna and other foods -- influences individuals' willingness to trust others.
They did this by giving one group of participants orange juice with added tryptophan and another group a placebo. Participants were then asked to play a game in which one person was given 5 euros and was allowed to decide how much money he or she would give to another participant throughout a series of rounds. The person who started with the money would receive more cash only if the "trustee" gave the money back in return, however.
The idea for the experiment came from previous research, which found that the neurotransmitter serotonin is known to play a role in cooperation among individuals.
"Mutual trust is an important condition for [cooperation]," Colzato said in a statement. "Society functions in the first place on the basis of mutual trust. After that, such institutions as the courts and the police come into play."
In the end, those who had taken tryptophan gave significantly more money than those who merely took the placebo.
"These results support the idea that 'we are what we eat': the food one eats has a bearing on one's state of mind," Calzato said. "Food can thus act as a cognitive enhancer that modulates the way one thinks and perceives the physical and social world. In particular, the intake of tryptophan may promote interpersonal trust in inexpensive, efficient, and healthy ways."