Monoamine Oxidase-A: The Gene That Controls Violent Behaviour?

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:

I couldn't find an image of the gene MAO-A, so here's a picture of an aggressive fist
I couldn’t find an image of the gene MAO-A, so here’s a picture of an aggressive fist

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. 


Why Criminals are Less Intelligent (Satoshi Kanazawa)

In a 2010 article for Psychology Today, Satoshi Kanazawa gives a suggestion as to why low IQs and criminality seem to go hand in hand:

“Criminologists have long known that criminals on average have lower intelligence than the general population, but they do not know why.  The Hypothesis may be able to shed new light on this question.

Controversial evolutionary biologist Satoshi Kanazwa
Controversial evolutionary biologist Satoshi Kanazwa

From the perspective of the Hypothesis, there are two important points to note.  First, much of what we call interpersonal crime today, such as murder, assault, robbery, and theft, were probably routine means of intrasexual male competition in the ancestral environment.  This is how men likely competed for resources and mating opportunities for much of human evolutionary history.  They beat up and killed each other, and they stole from each other if they could get away with it.

We may infer this from the fact that behavior that would be classified as criminal if engaged in by humans, like murder, rape, assault, and theft, are quite common among other species.  The criminologist Lee Ellis documented many instances of these “criminal behavior” among different species with photographs in 1998.  The primatologist Frans de Waal and his colleagues have documented brutal murders, assaults, and other interpersonal violence among chimpanzees, bonobos, and capuchin monkeys.

Second, the technologies and institutions that control, detect, and punish criminal behavior in society today – CCTV cameras, DNA fingerprinting, the police, the courts, the prisons – are all evolutionarily novel.  There was very little formal third-party enforcement of norms in the ancestral environment, only second-party enforcement (retaliation from vigilance by victims and their kin and allies) or informal third-party enforcement (ostracism).

It therefore makes sense from the perspective of the Hypothesis that men with low intelligence may be more likely to resort to evolutionarily familiar means of competition for resources (theft rather than full-time employment) and mating opportunities (rape rather than computer dating), and not to comprehend fully the consequences of criminal behavior imposed by evolutionarily novel entities of law enforcement.

Men with lower intelligence are less likely truly to comprehend evolutionarily novel entities.  Some of these evolutionarily novel entities are alternative means to resource acquisition and accumulation they could pursue instead of evolutionarily familiar means which are now classified as criminal in civilized societies.  Other evolutionarily novel entities they are less likely truly to comprehend are means that law enforcement agencies employ to detect and capture criminals.  The Hypothesis therefore offers one possible explanation for the negative association between intelligence and criminality.

At the same time, the Hypothesis also offers a novel hypothesis with regard to intelligence and criminality.  As I mention above, while formal third-party enforcement of norms is evolutionarily novel, second-party enforcement and informal third-party enforcement are evolutionarily familiar.  Thus the Hypothesis would predict that the difference in intelligence between criminals and noncriminals will disappear in situations where formal third-party enforcement of norms is weak or absent, and criminal behavior is controlled largely via second-party enforcement, such as situations of prolonged anarchy and statelessness, in fact, any situation that resembles the ancestral environment.  Paradoxically, the Hypothesis would predict that less intelligent men will commit fewer crimes if the police disappeared, although more intelligent men may commit more crimes then.”