The National Longitudinal Study of Adolescent Health, or Add Health, is an ongoing longitudinal study of a nationally-representative sample of more than 20,000 adolescents in grades 7-12 in the United States in 1994-95 who have been followed through adolescence and their transition to adulthood with four in-home interviews in 1995, 1996, 2001-02, and most recently in 2008-09 when they were aged 24-32. Embedded within the design of Add Health were oversamples of about 3,000 pairs of individuals with varying genetic relatedness, including MZ and DZ twins, full siblings, half-siblings, and adolescents with no biological relationship but who were raised in the same household. Because all design features of the Add Health Study (described below) relate to the sibling pairs subsample, Add Health sibling pairs are unique in that they are nationally representative, racially and ethnically diverse, and have comprehensive social, environmental, behavioral, and biological longitudinal data from early adolescence into adulthood.
