A central problem in modern biology is understanding how an individual’s traits, whether physical features, disease or behavior, result from the combined action of genes, the physical environment, and the social environment with which an individual interacts. Describing genetic influences on behavior is particularly challenging when genes carried by multiple social partners interact to generate behavior. The effects of a partner’s genes on an individual’s phenotype are termed indirect genetic effects (IGEs) and traits that are influenced by IGEs are termed “interacting phenotypes.” How interacting phenotypes, such as cooperation, evolve depends on both the genetic architecture of the behavior and the structure of the social environment. However, almost nothing is known about how often or to what degree IGEs influence an individual’s behavior, whether those effects vary across populations, nor what physiological mechanisms allow one partner’s genes to influence another partner’s behavior. Trinidadian guppies are an excellent model system to investigate indirect genetic effects on behavior because guppies perform a suite of cooperative antipredator behavior that is strongly influenced by social partners and varies greatly across populations depending on predation risk. My lab uses quantitative genetic breeding designs with wild fish and inbred tester strains that vary in cooperative antipredator behavior, along with measures of gene expression, hormone excretion, and sensory sensitivity to dissect the genetic architecture of IGEs.