Participants were originally recruited from the Colorado Longitudinal Twin Study (LTS), which consists of same-sex twins whose emotional, cognitive, and behavioral development has been studied since birth. The LTS twin families were originally identified through the Colorado Department of Health’s Division of Vital Statistics between 1986 and 1990. Twins were ascertained preferentially for normal gestational periods and birth weights, as well as residing within a 2-hr driving distance of Boulder, CO. The LTS sample is largely Caucasian; approximately 8% report ethnic minority status. Retention of the LTS sample has been high; approximately 420 of the original 483 families (87%) are still active in the longitudinal studies (for more information about the LTS sample, see Rhea, Gross, Haberstick, & Corley, 2006).
