To test these predictions, in a within-subject, placebo-controlled, double-blind, cross-over study (Experiment 2), the serotonergic system was manipulated by protracted oral administration of citalopram—a selective 5-HT reuptake inhibitor commonly used to treat depression (see STAR Methods for details and Table S1 for the testing schedule and the measurement of 5-HT levels in blood). The gene discussed is HTR5A; the disease is major depressive disorder.