The Asn40 SNP in the OPRM1 gene was assayed using a modification of restriction fragment length polymorphism procedures reported by Bergen et al. (1997). Samples were genotyped again using the ABI Taqman assay for rs1799971 to ensure that the high frequencies of the Asp40 variant found were not due to genotyping error. The 48-bp VNTR in exon III of the DRD4 gene was assayed using modifications of previously reported methods (Sander et al., 1997). Consistent with the existing literature, participants were grouped by OPRM1 status such that the Asp40 variant group was comprised of participants who were either heterozygous or homozygous for the Asp40 variant and the Asn40 group was comprised of those homozygous for the Asn40 variant. Participants were grouped by DRD4 status using conventional methods (Hutchison et al., 2002, 2003), with the DRD4-long group (DRD4-L) composed of individuals with at least 1 copy of the ≥7 repeat allele and the DRD4-short (DRD4-S) group composed of individuals who had neither copy greater than 6 repeats. The observed frequency of the DRD4 and OPRM1 genotype combinations were: DRD4-S and OPRM1
