How people feel is regulated by the brain. But mood is also influenced by other people. This current project aims to study how serotonin, an evolutionarily ancient neurotransmitter that modulates a wide range of functions, regulates interpersonal functioning in individuals at risk for major depressive disorder (MDD), a psychiatric disorder often characterized by irritability, rejection sensitivity, and social withdrawal. In Study 1, during transient serotonin depletion, participants complete a novel laboratory task that uses dynamic video stimuli to measure their perceptions of others' affective states. Study 2 uses ecological momentary assessment to measure, while participants are taking the serotonin precursor tryptophan, their affects, behaviours, cognitions, and perceptions of others' behaviours during every day social interactions.