Serotonin, a chemical that in the brain affects mood, also seems to control behavioral reactions to unfairness. Researchers at the University of Cambridge depleted the serotonin levels of volunteers in part by feeding them a cocktail of amino acids that lacked tryptophan, an ingredient that the body needs to make serotonin. The volunteers then played an ultimatum game, in which they had to decide whether to accept an offer by a “proposer” to split, however unevenly, a pot of money. Rejection of the offer meant that no one got anything. Lack of serotonin, the experimenters found, made refusal more likely—for instance, 82 percent of serotonin-deprived individuals refused a 20 percent piece of the pie, compared with 67 percent of normal subjects given the same ultimatum. Take it or leave it from the study published online June 5 by Science.