Gene-X-Environment interactions and biobehavioral development in rhesus monkeys and other macaques Stephen J. Suomi1 1Laboratory of Comparative Ethology, National Institute of Child Health & Human Development, National Institutes of Health Abstract Recent research has identified functionally equivalent polymorphisms in specific genes carried by both humans and rhesus monkeys that appear to interact with specific early experiences in influencing the development of a wide range of behavioral and biological functions. For example, the “short” allele of the serotonin transporter (5-HTT) gene is associated with increased risk for developing anxious and depressive symptoms in humans and delayed early neurobiological development, impaired serotonergic functioning, and excessive aggression and alcohol consumption in rhesus monkeys but only among individuals with aberrant early experiences (e.g., a history of child maltreatment in humans and early peer-only rearing in rhesus monkeys). Similar gene x environment interactions involving a polymorphism in the MAO-A gene, specific early experiences, and individual differences in aggressiveness have also been demonstrated in both humans and rhesus monkeys. However, genetic analyses of several other macaque species have revealed that although all of these other macaque species possess both the 5-HTT and MAO-A genes, they do not have any of the functional polymorphisms found in humans and rhesus monkeys, i.e., there is no within-species variability for these genes. Implications of these recent findings will be discussed.