Apart from MAO, only a few candidate genes have been linked to aggressive behavior to date. The candidate genes that have been found to be associated with aggressive behavior in humans have, in many cases, been replicated in animal studies. The majority of these candidate genes are genes of the dopamine, serotonin, and norepinephrine neurotransmitter systems. The dopa-mine system is involved in mood, motivation and reward, arousal, as well as other behaviors. The serotonin system is involved in impulse control, affect regulation, sleep, and appetite, whereas the epinephrine and norepinephrine system facilitate fight-or-flight reactions and autonomic nervous system activity (Niv and Baker, 2010). For example, dopaminergic candidates, including dopamine receptor DRD4, has been found to be involved in ADHD and externalizing behavior, and DRD2 has been found to be involved in substance abuse and disinhibition (Niv and Baker, 2010). The DRD3 polymorphism has been found to be associated with impulsivity. This association was significant in violent, but not in nonviolent individuals, and there were no association between DRD3 and violence per se (Retz et al., 2003). Dopamine transporter gene DAT1
