Our sample consisted of participants in the Child Development Project (CDP), who were originally recruited through kindergartens at three cities in the United States in 1987 and 1988. CDP participants represented the broader population demographically and behaviorally, as determined by teacher and sociometric ratings of the entire populations at those sites. The original CDP sample consisted of 585 children (52% male and 48% female; 81% European American, 17% African American, and 2% belonging to other ethnic groups). Data collections began the summer before the participants entered kindergarten (at about 5 years of age); follow-ups have been conducted annually and are ongoing, with a 90% continued-participation rate in the early-adulthood assessments. DNA was collected from CDP participants via saliva sample using Oragene collection kits under the supervision of a specially trained interviewer. Saliva samples were de-identified and mailed to Washington University in St. Louis, Missouri, where DNA extraction and genotyping occurred. The DNA samples were obtained from 452 individuals, representing 93% of the target subset of regular CDP participants. The institutional review boards at all sites approved the study.
