DNA was collected by buccal swabs and extracted using a modified phenol-chloroform technique (Freeman et al., 2003). A portion of the collected DNA was genotyped for the polymorphic VNTR site in the DRD4 gene at the Penn State Genomics Core (Anchordoquy, McGeary, Liu, Krauter, & Smolen, 2003) using primer sequences developed by Lichter et al. (1993) with the forward primer fluorescently labeled. Amplification products were analyzed using a 3730XL DNA Analyzer and Genotyper software, Version 4.0 (Applied Biosystems, Foster City, CA). Nine alleles were detected, ranging in size from 2 repeats to 10 repeats. Frequencies of the most common alleles (> 5%) were 2-repeat (9%), 4-repeat (64%), and 7-repeat (20%). The remaining alleles (3-, 5-, 6-, 8-, 9-, and 10-repeats) summed to 3%. A total of 23 distinct genotypes were detected, the five most common being 7/7 at 3.9%, 3/4 at 4.1%, 2/4 at 12.8%, 4/7 at 27.4%, and 4/4 at 39.3%. Regenotyping ~10% of the samples revealed an error rate of 7.5% (4/53). For the present study, DRD4 variability was coded on the basis of the presence (7+) versus
