Adolescents’ DNA was obtained using Oragene™ DNA kits (DNA Genotek; Kanata, Ontario, Canada). Adolescents rinsed their mouths with tap water and then deposited 4 ml of saliva in the Oragene sample vial. The vial was sealed, inverted, and shipped via courier to a central laboratory in Iowa City, where samples were prepared according to the manufacturer’s specifications. Genotype at DRD4 was determined for each adolescent as Lichter et al. (1993) described. This approach involved using the primers F-CGCGACTACGTGGTCT ACTCG and R-AGGACCCTCATGGCCTTG, standard Taq polymerase and buffer, standard dNTPs with the addition of 100 μM 7-deaza GTP, and 10% DMSO. The resulting PCR products were electrophoresed on a 6% nondenaturing polyacrylamide gel and the products visualized using silver staining. Genotype was then called by two individuals blind to the study hypotheses and other information about the participants. None of the alleles deviated from Hardy-Weinberg equilibrium (p = .062, ns). For tests of the G×E hypotheses, DRD4 status was dummy coded; participants carrying at least one l allele were assigned a code of 1 (48.28% of the sample), and participants carrying two
