ABSTRACT
One of the most curious facts about psychological studies of parents and children is that nearly all investigations focus on differences among families. The vast literature on social class, socialization, child rearing styles, and parent-child interaction are based almost exclusively on samples of one child per family. Explanatory variables in these traditions refer nearly always to family or parental characteristics that are shared by children in the same family and thus cannot explain why siblings differ. This fact is curious because most behavioral differences among people are found among siblings within families.
