The paper analyzes changes in conceptions of social interaction in infancy from a unidirectional process to a bidirectional one requiring dyadic analysis. Developments in the fields of ethology and comparative psychology are cited as are implications of early behaviorist research that pointed out the effects on the mother of the infant's behavior. The added dimension of interaction in bidirectional models is discussed. Additional input from personality theory regarding the importance of social contact on the development of the infant is noted. The notion of pre-adaptedness or preparedness for interaction is briefly cosidered, as are the effects of changes in the mother's behavior during the first year of her infant's life. (CL)