Sex-specific social decision-making is regulated by monoamine signaling. A. Male Drosophila typically respond with aggression towards other males, though they will sometimes attempt to court them. This decision between aggressive and courtship responses to same-sex conspecifics is regulated by sex-specific octopamine signaling. Both loss of the ability to synthesize octopamine, and genetic feminization of octopaminergic neurons, results in males that court other males with increased frequency (adapted with permission from . Copyright 2007, The National Academy of Sciences of the USA). B. Male mice respond to other males with aggression much more frequently than courtship or mating behaviors. This decision to respond to other males with aggression, rather than mating, is regulated by serotonin signaling. Tph2 mutant male mice defective in serotonin synthesis have dramatically increased frequency of mating behaviors directed towards other males. This behavior can be partially rescued by treating animals with the serotonin precursor 5-HTP, bypassing the requirement for the Tph2 gene in serotonin synthesis (adapted with permission from . Copyright 2011, Macmillan Publishers Ltd.).