If you find yourself a little drowsy after lunch, the culprit could be the tryptophan in your turkey sandwich. But the next time you sit down to a Thanksgiving feast (or a table laden with other foods high in tryptophan), you might wonder why you’re also filled with extra goodwill.
Experimenters investigated tryptophan’s influence on interpersonal trust by administering oral doses of tryptophan (or a placebo) to pairs of strangers in the laboratory.1 (In this study, tryptophan and the placebo were administered in powdered form, dissolved in orange juice—now that would be an unexpected mix-in for your breakfast beverage!) The strangers completed an unrelated task together and then played the “Trust Game.” This “game” is actually a lab task, which determines the extent to which someone (the truster) trusts someone else (the trustee) by measuring the amount of money the truster transfers to the trustee.
In the Trust Game, the truster receives a sum of money (five euros in this case) and can share any amount with the trustee. The amount the trustee receives is tripled, and the trustee may opt to share this extra wealth with the truster. Thus, if Chris, the truster, gives four euros to Kat, then Kat ends up with 12 euros that she can keep for herself or share with Chris. If Chris doesn’t trust Kat to share, Chris might give her less cash at the start—but this distrust could mean losing the chance to make more money.
In this study, the strangers sat at separate cubicles and believed that one of them would play the truster while the other would play the trustee. However, both strangers played the role of truster and did not actually interact with each other during the game. The experimenters discovered that strangers who received the dose of tryptophan were willing to transfer more money to the person they assumed was the trustee.
So readers, if your slick Uncle Dave (or a sketchy-looking Santa at the mall) asks if you can spare a few dollars, your answer may depend on when you last chowed down on some turkey* leftovers, or even on your last Starbucks order—there’s tryptophan in milk and chocolate, after all! (“Barista, make mine with two sugars and a shot of tryptophan…”)
*Vegetarians are not exempt, as soy protein, spinach, and many other plant-based foods contain more tryptophan than turkey. Better watch out for that Tofurkey!
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1Colzato, L. S., Steenbergen, L., de Kwaadsteniet, E. W., Sellaro, R., Liepelt, R., & Hommel, B. (in press, 2013). Tryptophan promotes interpersonal trust. Psychological Science. doi: 10.1177/0956797613500795
Dr. Helen Lee Lin – Science of Relationships articles | Website/CV
Helen’s past research has focused on potential problems in relationships, such as keeping secrets from a significant other. She is also interested in communication as well as the use and consumption of media in relationships, and is planning to work in applied contexts for her future projects. 
