Monoamine oxidase A (MAOA) is an enzyme that breaks down important neurotransmitters in the brain, including dopamine and serotonin; low levels of serotonin have been associated with impulsive and aggressive behaviour. MAOA is regulated by the MAOA gene and humans have various forms of the gene, resulting in different levels of activity of the enzyme. One variant of the gene is associated with high levels of MAOA (MAOA-H) and another variant is associated with low levels (MAOA-L). Several studies have now found a correlation between the low-activity form of the MAOA gene and aggression.