From answers provided in the in-school survey, Add Health drew supplemental samples based on the genetic relatedness of siblings in a household. If an adolescent indicated that he or she was a twin in the in-school survey, they were selected with certainty (e.g., 100%) for inclusion in the in-home Wave I sample. Full siblings occur naturally in large numbers in the core in-home sample, but half-siblings and unrelated adolescents (e.g., stepsiblings, foster and adopted children, adolescents in group homes) who participated in the in-school survey (i.e., were in grades 7-12 in 1994-95) and lived in the same household were also oversampled. Table 1 shows the number of pairs of adolescents in the sibling pairs subsample who were interviewed in home at Wave I. This subsample includes more than 3,000 pairs of adolescents who have varying degrees of genetic relatedness and represent a fully articulated behavioral genetic design. These pairs of adolescents took the same questionnaires, share the same home environment, and share, in most cases, the same school and neighborhood environment. The embedded sibling pairs design in Add Health and
