A major challenge to current ideas on the nature of familial influences on individual development has been posed by evidence from developmental behavior genetics. Children who grow up within the same family—siblings who share 50% of their segregating genes, the same parents, and the same family environment—develop to be strikingly different in personality, in adjustment, in confidence and self-esteem, and in psychopathology. Such differences between siblings have been documented in an extensive range of studies by behavior geneticists, and they present the challenge to those interested in the impact of parent-child relationships on individual development (Dunn & Plomin, 1990; Plomin & Daniels, 1987; Rowe & Plomin, 1981; Scarr & Grajek, 1982). What the data from studies of siblings, adoptive siblings, and twins show us is that the sources of environmental influence that make individuals different from one another work within rather than between families. To understand the salient environmental influences we have to be able to explain what makes two children different from one another within the family. The factors that are shared cannot per se be the environmental influences that exert significant effects on children.