Early evidence for the role of MAOA on human aggression comes from a Dutch kindred in which some males lacked a functional copy of the gene and exhibited mild mental retardation along with aggressive and violent behavior (Brunner et al., 1993). In these men, MAOA was effectively knocked out, since the gene is on the X chromosome and males are hemizygous, having only one copy of the X chromosome. High levels of aggression are also observed in knock-out animal models (Cases et al., 1995).
