It is a common article of belief that each child is unique. Action based on this belief, though, is rare. Researchers have largely neglected the question of the causes of children's individual uniqueness. But, when difficulties and serious problems arise in the course of child rearing, causality is located in the dynamics of family functioning. Recent research on various problems suggests that such a narrow functionalist view of causation is inadequate and reasserts the existence and importance of individual differences and their effects. Individual differences in children have been investigated by Thomas, Chess, and Birch (1956, 1968, 1977), who identified individual characteristics of 1-month-old infants and explored the extent to which those characteristics remained constant and influenced later psychological development. The researchers identified three temperament styles: the easy, difficult, and slow-to-warm child. Inheritability of individual characteristics of personality was researched by Buss and Plomin (1975) who found support for a genetic component in the temperament dimensions of emotionality, activity, sociability, but none for impulsivity. Both sets of authors advance an interactionist model of the relation between temperament and environment; other related studies support the interactionist view. Taken together, findings suggest specific ways that differences in children's temperament affect caretaker-child relationships and differences in parents' temperament affect their initial parenting practices. (RH)