The Nonshared Environment in Adolescent Development (NEAD) project: a longitudinal family study of twins and siblings from adolescence to young adulthood.

The Nonshared Environment in Adolescent Development (NEAD) project is a longitudinal study of twins/siblings and parents that has been assessed 3 times: middle adolescence, late adolescence and young adulthood (N=720 families at Time 1). Siblings varied in degree of genetic relatedness including identical twins, fraternal twins, full siblings, half siblings and genetically unrelated (or step) siblings. There were also two family types: nondivorced and step. A multimeasure, multirater approach was taken in NEAD, with data collected from all participants (2 twins or siblings, mother and father) as well as from coded videotaped observations of family interactions. Detailed assessments of family relationships, adolescent adjustment and competence were collected at all 3 times. The original aim of NEAD was to identify systematic sources of nonshared environmental influences that contribute to differences among family members. Although systematic sources of nonshared environmental influences were not found in NEAD, three major sets of findings emerged: (1) genetic influences on family relationships and on associations between family relationships and adolescent adjustment; (2) genetic and environmental influences on adolescent adjustment, comorbidity and stability and change in adolescent adjustment from middle to late adolescence; and (3) genetic influences on relationships outside the family.