In some very real sense violence is embodied in the human genetic/evolutionary legacy as is evidenced by its frequent manifestation in the human species as well as in others. Although the complex outcomes of the interactions of genetic predispositions with the social, ecological and cultural environments are themselves not embodied in the genetic makeup, nevertheless any violent species-specific behaviour is part of its evolutionary heritage to the extent that any such cultural heritage forms a continuum with the biological heritage.
In 1993, researchers from the Netherlands and the USA reported that in one large family they have linked a small genetic defect with impulsive, hostile and violent behaviour in the males. Those afflicted often reacted to the most mildly stressful occasions with aggressive outbursts, shouting at, cursing or assaulting the person they deemed a threat. At other times the men have committed arson, attempted rape and exposed themselves in public. In addition, their intelligence is on the low end of normal, with an average IQ of around 85 to 90. The abnormal behaviours are linked to the gene responsible for the production of monoamine oxidase-a, an enzyme which destroys neurotransmitters in the brain.