frequency of the 118G allele was considerably lower, i.e. 11.1% in unaffected controls and 13.6% among alcohol-dependent individuals (Bergen et al., 1997). There are no explicit explanations for such big discrepancies between the studies. However, in the study of Bergen et al., the study population consisted of 324 males of which 165 were criminal offenders undergoing forensic psychiatric examination and 159 were volunteers recruited through newspaper advertisement. Eighty-eight individuals belonging to both criminal offenders and control groups were diagnosed as alcohol dependent. As several studies have suggested an association between the OPRM1 polymorphism and personality traits (Corley et al., 2008; Luo et al., 2008; Troisi et al., 2011), a study where both alcohol-dependent and control populations are largely composed of criminal offenders may exhibit distorted genotype distributions.
