Sibling differentiation in adolescence: Implications for behavioral genetic theory

The presence of sibling "differentiating processes"-defined as processes in which increased sibling similarity in environmental or genetic factors lea ds to differences in sibling outcomes-poses a challenge for standard behavi oral genetic theory and research. The presence of differentiation processes may affect estimates of genetic and environmental parameters in ways that have not been fully recognized. Utilizing data from the Nonshared Environme nt and Adolescent Development project, this study examined whether differen tiating processes existed for seven composite indices of positive and negat ive adolescent adjustment. The 720 sibling pairs in the study were broken d own into groups by age difference (0-4 years) between siblings. The hypothe sis that siblings close in age would demonstrate lower correlations on adju stment measures was generally supported at two time points, three years apa rt. However, siblings one year apart at Time 1 were more similar to each ot her than were siblings two years apart, suggesting that shared environmenta l influences counteract sibling differentiation processes for these sibling s. The overall trend supporting sibling differentiation was found to be unr elated to measures of sibling positivity and negativity.