Participants were drawn from a large sample of 2484 (1148 males, 1336 females) respondents between the ages of 17 and 29 who participated in the second wave of a longitudinal study at the NIDA funded Center on Antisocial Drug Dependence at the University of Colorado. Twins were drawn from the Colorado Community Twin Study and Longitudinal Twin Study at the University of Colorado; Rhea and colleagues (2006) provide details regarding the recruitment of each sample. The sample was predominantly Caucasian (87%) and had an equivalent number of male and female participants at each age. The average age of participants in the study was 20 years (standard deviation (SD) = 2.59). There were a total of 681 monozygotic (MZ) female twins (which included 23 incomplete pairs), 415 dizygotic (DZ) females (which included 21 incomplete pairs), 542 MZ males (which included 16 incomplete pairs), 375 DZ males (which included 27 incomplete pairs) and 471 twins from opposite sex (OS) pairs (which included 33 incomplete pairs). Zygosity was determined using a nine-item questionnaire and 11 highly informative DNA markers. Twin number was randomly assigned to each individual within a family unit.
