The authors examined factors that could explain the higher levels of psychosocial well being observed in past research in mothers of individuals with Down syndrome compared with mothers of individuals with other types of intellectual disabilities. The authors studied 155 mothers of adults with Down syndrome, contrasting factors that might validly account for the ?Down syndrome advantage? (behavioral phenotype) with those that have been portrayed in past research as artifactual (maternal age, social supports). The behavioral phenotype predicted less pessimism, more life satisfaction, and a better quality of the mother?child relationship. However, younger maternal age and fewer social supports, as well as the behavioral phenotype, predicted higher levels of caregiving burden. Implications for future research on families of individuals with Down syndrome are discussed.