The effect of tryptophan on quarrelsomeness, agreeableness, and mood in everyday life

In humans there is an association between low serotonin levels and aggressive behavior, and drugs that increase serotonin function have been used to treat aggression. In monkey alterations in serotonin function seem to influence behavior along the agonistic-affiliative axis, with increases in serotonin function not only decreasing aggression but also increasing the grooming of other animals. Measurement of human social behavior in everyday life has advanced to the stage where it is possible to measure behavior along the agreeable-quarrelsome axis. Therefore we performed two double-blind cross-over studies to compare the effects of tryptophan, the serotonin precursor, with placebo on the social behavior of healthy people. In the first study tryptophan decreased quarrelsome behaviors. In the second study on irritable people tryptophan not only decreased quarrelsome behaviors but also increased agreeable behaviors and improved mood. Our studies suggest that increasing serotonin synthesis with tryptophan moves behavior along the axis that encompasses aggressive, quarrelsome, and agreeable behaviors towards more positive social behavior in healthy people in everyday life. Irritability is a risk factor for various mental and physical disorders. It remains to be seen whether increasing serotonin function in irritable people improves their mental and physical health.