QUOTE : Normal volunteers played the Responder in the Ultimatum Game twice, either after consuming a tryptophan-depleting drink (which lowers serotonin levels in the brain; see Chapter 7) or a control beverage.22 During the tryptophan depletion, Responders couldn’t accept a deal; they showed increased impulsivity and vindictiveness, which was predictable based on the change in tryptophan levels in their blood. This shows that reducing the molecule of contentment biochemically resulted in more impulsivity and spite behaviorally. Even giving tryptophan to an ornery dude can acutely improve his mood.