Subjects were healthy male social drinkers, consuming less than 20 standard drinks per week. They were non-smokers who had never smoked, or had quit at least a year prior to enrolling in the study. Because the ventral striatum shrinks an average of 3.6 to 3.7% per decade 17, the age range was restricted to 21–45 years in order to reduce age-related variability. Furthermore, because rhesus macaque OPRM1 genotype predominantly modulates psychomotor responses to alcohol in males 14, and because recent human data indicate only borderline detectable striatal DA release in response to alcohol in females 18, the study population was restricted to males. Subject demographics are provided in table S1. Allele frequencies of OPRM1 118G in Caucasian populations are around 15% 19, and 118G homozygotes are rare. Subjects were recruited into two approximately equally sized groups: 1) individuals homozygous for the major 118A allele (118AA genotype; n=16) of the OPRM1 polymorphism; 2) individuals carrying one or two copies of the variant 118G allele (118AG or 118GG genotype; because no subject with the rare 118GG genotype was in fact found, this
