This project utilized twins to study differential mother-sibling interactions. The use of twins circumvented the traditional confounds of studying siblings of different ages or at two points in time. When the twins were 7 and 9 months of age, mothers spent 2.5 min alone with each infant in an attempt to elicit child vocalizations. The mother and infant behaviors were coded both microanalytically and globally. The infant attention behaviors were influenced primarily by unique environment, whereas the temperament behaviors were influenced by both unique environmental and genetic effects. Mothers tended to treat both children similarly, regardless of zygosity, suggesting that maternal characteristics drove the mother-infant interactions. Thus, even though identical twins were more similar on some measures than fraternal twins, mothers tended to treat both types of twins comparably regardless of infant characteristics or behaviors.