Because MAOA is an X-linked gene, male subjects were straightforwardly assigned to one of two genotype groups: 1) subjects carrying one low activity allele (2-, 3-or 5-repeat form), and 2) subjects carrying one high activity allele (3.5-or 4-repeat form). Females were assigned to three genotype groups: 1) homozygous subjects carrying two low activity alleles, 2) homozygous subjects carrying two high activity alleles, and 3) heterozygous subjects carrying one low and one high activity allele. Expression studies have demonstrated that in female skin fibroblasts the MAOA gene undergoes X-inactivation and shows mono-allelic expression [21]. Although the extent of X-inactivation in human brain is not known, these findings suggest that heterozygous low-high females are mosaic for different MAOA alleles and are likely to have intermediate enzymatic activity. A functional discrimination analysis showed that in our sample the three female genotypes were indistinguishable by physical aggression scores of the Aggression Questionnaire (Wilk's λ = 0.99; F2,170 = 0.74, P = 0.47; AQ-PA, see Materials and Methods [22]) and thus, for the purposes of our analysis, we grouped heterozygous female participants together with low-low
