Increasing evidence suggests the importance of heritable factors in the development of aggressive behavior (Burt, 2009; Miles and Carey, 1997; Rhee and Waldman, 2002). The first study that showed a link between a specific genotype and aggressive behavior examined the genetic material of members of a large Dutch family. This specific family had for decades been found to be prone to violent, aggressive, and impulsive behavior, including fighting, arson, attempted rape, and exhibitionism. Some of the male family members were also intellectually disabled. The aggressive males in this large family were shown to share a mutation in the gene that codes for the enzyme MAO (monoamine oxidase A). MAO breaks down brain chemicals (neurotransmitters) such as serotonin, nor-adrenaline, and dopamine, which transmit messages from one nerve cell to the next. In the afflicted males, however, a mistake in the coding sequence governing proper production of MAO was detected. As a result, abnormally large quantities of these neurotransmitters were found in the blood of the affected males (Brunner et al., 1993). Although this genetic defect remains the first such link to aggressive behavior in humans, exactly how the genetic defect causes aggressive, impulsive behavior, or mental retardation is not known.
