The behavioral data used in the present study are part of the National Longitudinal Study of Adolescent Health (Add Health). Add Health is a longitudinal study of a nationally representative sample of adolescents that were in grades 7–12 in the United States during the 1994–95 school year. The current analyses utilized data from respondents in the sibling-pairs subsample for which phenotypic and DNA were available. Details about the Add Health data are provided at http://www.cpc.unc.edu/projects/addhealth and in Harris et al. 2006. Study protocols were approved by Institutional Review Boards (NC and CO, USA), and subjects provided written informed consent.
