Genotyping was conducted at the Georgia Genomics Facility (GGF) at the University of Georgia (http://dna.uga.edu) where DNA was extracted from saliva samples according to the manufacturer's specifications by a technician blind to other sources of data for the research project. Genotyping was performed on the DRD4 polymorphism of 48-base pair variable number of tandem repeat (VNTR) in exon 3 as described by LaHoste et al. (1996). The genotype at DRD4 was determined for each participant using the primers forward 5'-CGA CTA CGT GGT CTA CTC G-3' and reverse 5'-/56-FAM/AGG ACC CTC ATG GCC TTG-3', ABI 360 PCR master mix plus 10% GC enhancer. Determination of the allele length was performed by analysis on an automated capillary sequencer (AB13730, Applied Biosystems). Genotype was then called using GeneMapper 4.0 software (Applied Biosystems) by an individual blind to the study hypotheses and other information about the participants. Quality control procedures included a genotype call rate of GE 98%, subject filtering call rates of GE 95%, non-significant departures of Hardy-Weinberg equilibrium, and outlier screening (using a criterion of 5 SDs). None of the alleles
