Making a business deal or breaking up with your partner over dinner are relatively common practices. That’s because closing a deal over a meal fosters feelings of trust, while being on the verge of breaking up with someone creates the illusion of trust. Now, a new study has found that these feelings of trust might be real, and that they’re produced by the amino acid tryptophan.

Tryptophan is commonly found in cheese, chicken, eggs, fish, milk, and soy. It’s an essential amino acid, meaning that your body cannot produce it, but needs it from your diet. It helps produce niacin, which is involved with converting food to energy, as well as serotonin, the neurotransmitter associated with feelings of well-being and happiness.

Leiden University psychologist Lorenza Colzato and her team wanted to see if tryptophan could foster feelings of mutual trust via serotonin production. “Mutual trust is an important condition for cooperation,” Colzato said in a press release. “Society functions in the first place on the basis of mutual trust.”

The researchers performed a test on two groups of participants, of whom one group had consumed orange juice with added tryptophan while the other group had a placebo. The test measured how much the participants, known as the trustees, trusted another participant, known as the trustor. Trustees were given five euros and told that they could either keep the money, or give it to the trustor, who would then receive extra money, depending on the amount given, and then decide how much he wanted to give back to the trustee — sometimes this amount was more than five euros. Mutual trust was measured by the amount the trustees gave the trustors.

The researchers found that participants who had taken the tryptophan-filled orange juice gave more money to the trustors than participants who drank to placebo. “These results support the idea that ‘we are what we eat,’” Colzato said in the release. “The food one eats has a bearing on one’s state of mind. 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.”

The researchers’ ultimate goal in this study was to explore different ways in which cooperative behavior could be encouraged. The trick to cooperative behavior is to accomplish mutual trust, Colzato said in a video that accompanied the study. “After all, most people will only work together if they expect others to do so as well.”

Source: Colzato L, Steenbergen L, Hommel B, et al. Tryptophan Promotes Interpersonal Trust. Psychological Science. 2013.