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We observed the effects of manipulating 5-HT function on behavior in the Ultimatum Game, where players must decide whether to accept or reject fair or unfair monetary offers from another player.

Participants with depleted 5-HT levels [by reducing tryptophan in their diet for a short time] rejected a greater proportion of unfair, but not fair offers, without showing changes in mood, fairness judgments, basic reward processing, or response inhibition. [Normally, players tend to reject about 20% to 30% of the unfair offers. But the rejection rate for the serotonin depleted players was more than 80% - interpreted as increased expression of aggressive social responses.]

Our results suggest that 5-HT plays a critical role in regulating emotion during social decision-making.

Source: Science, online June 5, 2008. DOI: 10.1126/science.1155577, by Crockett MJ, Clark L, Tabibnia G, Liegerman MD, Robbins TW. Department of Experimental Psychology and Behavioral and Clinical Neuroscience Institute, University of Cambridge, UK; Department of Psychology, University of California Los Angeles, USA. [E-mail: Molly J. Crockett mc536@cam.ac.uk]