Genetic variants such as that in the Dutch family are quite rare; however, a variable number tandem repeat (VNTR) polymorphism located ~1.2 kilobases upstream of the coding region of MAOA (in the promoter region) is common and is associated with variation in transcript levels. Alleles of the VNTR with 2, 3, or 5 repeats are associated with lower levels of MAOA mRNA than are alleles with 3.5 or 4 repeats (Sabol, Hu, & Hamer, 1998). The relevance of this polymorphism to behavior has recently become the focus of much research after the finding that the MAOA uVNTR (for upstream variable number tandem repeats) moderates an association between childhood maltreatment and behavioral outcomes in adulthood (Caspi et al., 2002). Caspi and colleagues found that males hemizygous for low-activity alleles were more likely to be disposed toward violent behavior, have diagnoses of ASPD, or have violent crime convictions, than were males hemizygous for high-activity alleles. Importantly, this effect was observed only under conditions of childhood maltreatment, such that males with low-activity alleles were more sensitive to the detrimental effects of maltreatment than
