Although many forms of aggressive behavior serve crucial functions in preserving the structure and social order of non-human primate groups, in some contexts aggression becomes prolonged, escalated, and intensified to the degree that it has far reaching deleterious consequences on the physical health, social life, and psychological well-being of socially housed laboratory primates. Sensitivity to laboratory animal well-being is passionate and intense in the public domain, and animal welfare groups make the claim that many conditions of captivity increase traumatic aggression. In the NIH/NCRR sponsored SPF breeding programs of rhesus and pigtail macaques, multimate-multifemale social housing in large outdoor corrals stimulates the natural social and environmental features characteristic of the species, enhancing their reproductive vigor and their psychological well-being. Despite this, one of the most vexing problems of socially housed macaques is their unpredictable propensity for exhibiting spontaneous, deleterious aggression. The proposed research therefore aims to investigate the factors responsible for the expression of deleterious aggression in outdoor, corral housed rhesus and pigtail macaques. Through a collaboration of experiences scientists from several institutions that maintain state-of-the-art laboratories, the methodology incorporates the most recent advances in the study of the relationship between behavior and genetics, brain monoamines and metabolites. brain androgens (and possibly estrogens), peripheral gonadal hormones and immunological measures. Intensive sampling of these physiological measures will be combined with computerized behavioral data collection of deleterious aggression, affiliation, post-conflict reconciliation, and sexual behavior. Through a series of longitudinal, multivariate comparisons of individuals, gender, groups, social contexts, physical environment, seasonal, and maturational factors in two closely related macaque species, this research aims to predict individuals and/or groups who are at risk for the expression of deleterious aggression. As predictor variables for deleterious aggression are discovered , this research also proposes to develop and scientifically document the incorporation of these findings into sound management practices that will enable colony managers to predict, prevent, to minimize harmful and traumatic aggression, thus optimizing the psychological well-being and reproductive performance of their socially housed breeding colonies.