There has been much talk over the years about a gene called Monamine Oxidase-A (MAO-O for short). This gene helps to limit the activity of neurotransmitters (brain chemicals) such as dopamine, adrenaline and serotonin. It has been found that people with low levels of MAO-A are more suceptable to the effects of these neurotransmitters and are thus more prone to aggressive and depressive behaviour. The following well-written article from a blog called “The Unsilenced Science” includes the following useful discussion:
“This gene produces an enzyme that breaks down the neurotransmitters that activate many of the brain’s circuits. Back in the 1990’s, scientists discovered a mutation of MAOA that completely turns it off. The result is called Brunner syndrome, which only 14 related men have been known to have (plus animal models). What makes this extremely rare mutation so important is that it proves that MAOA really is a violent delinquency gene. A man with Brunner syndrome is what expert psychiatrists refer to as “a bad guy.” Five of these men were arsonists. Some of the men were rapists or attempted killers. Inexplicably, four men with the mutation somehow escaped having the syndrome, but for men with the mutation, those are not good odds. Nevertheless, out of the approximately three-and-a-half-billion men in the world, 14 really are not that many. Trust me, your neighbor—the one with the motorcycle—is probably not one of them. In conclusion, out of sight, out of mind.
The MAOA gene has a portion with repeated segments of DNA. This section of the gene is called a promoter because having more repeats increases the amount of enzyme that the gene produces (with a rare, debatable exception). After a 2002 study found that having three repeats together with having suffered child abuse is somewhat associated with violent tendencies, a flood of follow-up research ensued, and MAOA was relabeled “the warrior gene.” This version of the gene and one with four repeats are the most common versions, or alleles. These studies always had a few people with neither the 3-repeat nor the 4-repeat allele. A small number only had 2-repeats. The scientists decided that having 2-repeats in the promoter is sort of like having 3-repeats, so they invented the term “MAOA-L.” (“L” stands for low. Pretty clever, huh?) However, a pair of studies in 2008 found that the 2-repeat allele is associated with twice the rate of violence without child abuse coming into the equation. This allele is less powerful than Brunner syndrome but far more common.
Two small studies gave hints that the especially dangerous 2-repeat allele might be more common among African Americans. One study wrote that 6% of their non-white (but probably mostly African-American) male subjects had this allele. The other had 5 of 37 (14%) African-American men possessing “rare MAOA alleles.” Those percentages are remarkable given that studies of white men have suggested that 1% or fewer have this gene. ”