The inclusion of hormones in social and personality psychology research offers researchers a powerful methodological tool, although careful consideration must be paid to the complex and dynamic biological systems that connect hormones to behavior. Testosterone has been clearly established as a biological fuel for dominance, although behavioral findings have been inconsistent and weak. We suggest a number of physiological systems that affect the influence of testosterone on behavior. Vasopressin synthesis and the aromatization into estradiol both serve to facilitate testosterone’s effects. Testosterone’s behavioral responses to threats are suppressed by basal cortisol and moderated by allele length on the serotonin transporter gene. Variation in the expression of monoamine oxidase A regulates the levels of neurotransmitters responsible for impulse control, potentially suppressing testosterone’s influence over behavior. We argue that researchers must consider these dynamic relationships between hormones and other biological systems when engaging in future social endocrinology research.