The Significance of Differences in Siblings 'Experiences Within the Family: Judy Dunn and Clare Stocker
What we know of the influences of family environment on children-for instance, the effects of parents' child-rearing attitudes, their mental health, the quality of the marital relationship, and maternal educational level-comes from studies that have compared the development of individual children growing up in different families. But the idea that this is the appropriate way to study family influences has recently been seriously challenged. Developmental behavioral geneticists have drawn our attention to a striking fact: Siblings brought up in the same family differ from one another on standard measures of personality and psychopathology, and to a lesser extent cognition, nearly as much as do unrelated children raised in separate families (Plomin & Daniels, 1987; Rowe & Plomin, 1981; Scarr & Grajek, 1982). That is, children who share 50% of their segregating genes and a common family environment develop into individuals who are not at all similar. The variables that we have assumed to be important are shared by these children, and yet they grow up to be so unalike. How can we account for these differences? And how can we best study their implications? These findings suggest that a new approach to investigating children's development is needed. Instead of studying between-family differences and assuming that all family variables are part of both siblings' shared environment and thus affect them equally, we need to investigate the microenvironments experienced by children within the same family. In doing so, we may be able to identify environmental factors that are not shared by the siblings, and that contribute to their differential development. A range of possible sources of such nonshared influence that affect children within the same family have been set out by Rowe and Plomin (1981): These include differential parental treatment, differences in the behavior that each sibling directs towards the other, or in what is experienced
within the relationship by each, differences in their experiences outside the family, with peers and teachers, and nonsystematic, idiosyncratic influences such as accidents or illnesses that affect one child but not the sibling. Any or all of these may contribute to the differences between siblings; it is an empirical matter for us to examine which are important at which stage of the life cyclefor which aspects of individual development.