Variation in the serotonin transporter gene‐linked polymorphic region (5‐HTTLPR) has been associated with anxiety and harm avoidance and is weakly associated with a number of neuropsychiatric disorders, including Type II alcoholism, which has a high rate of comorbidity with antisocial personality disorder. Studies have also demonstrated interactions between 5‐HTTLPR variation and environmental stress on the incidence of depression. As in humans, there is a serotonin transporter gene promoter length polymorphism in rhesus macaques that produces similar decreases in transcriptional efficiency. Macaques with histories of early‐life stress have been shown to exhibit impulsive aggression, incompetent social behavior and increased behavioral and endocrine responsivity to stress. In this paper, we review studies performed previously in our lab and present preliminary data examininng interactions between early rearing and serotonin transporter gene promoter variation on the incidences of play behavior and aggression in infant rhesus macaques. The data presented here highlight the importance of considering gene‐environment interactions when studying childhood risk factors for aggression, anxiety and related neuropsychiatric disorders and support the use of the nonhuman primate for studing gene by environment interactions in behavioral research.