The National Longitudinal Study of Adolescent to Adult Health (Add Health) is a nationally representative longitudinal study including over 20,000 adolescents originally sampled in Grades 7-12 in the United States between 1994 and 1995. Add Health respondents have been followed through adolescence and into early adulthood with four in-home interviews (1995, 1996, 2001-2002 and 2008-2009). The Add Health design included the oversampling of approximately 3,000 pairs of individuals who were raised in the same household. These pairs of individuals are biologically related to varying degrees including monozygotic (MZ) and dizygotic (DZ) twins, full siblings, half siblings and unrelated. For further details on the study design and sampling scheme for the Add Health Sibling Pairs Sample, including phenotypic, environmental and biological assessments, see Harris et al. (2013). During the fourth in-home visit (Wave IV; 2008-2009), Add Health collected saliva on the entire sample of Add Health respondents (N=15,701), including the sibling pairs subsample. Consent rates (consent to provide saliva for DNA extraction) among the sibling pairs subsample for Wave IV saliva collection was an impressive 96%, which was similar to the
