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The MAO-A gene, also known as the "Warrior gene", is thought to cause increased antisocial and violent behavior patterns in people who have experienced an abusive childhood. [ref] Caspi et. al. found similar results.

I also remember an episode of BBC Horizon (48x05) that studied psychopathy, which touched on the MAO-A gene and its links to anti-social behaviour, psychopathy and murder.

In popular media (e.g. the character Dr Raymond Langston in CSI: Crime Scene Investigation[ref]) people with a mutation in this gene have been depicted as short-tempered and innately violent.

The results seem pretty solid around violent behaviour - a combination of a poor childhood, especially an abusive one, and an abnormal MAO-A gene can lead to violent behaviour and psychopathy.

I recently saw someone present a quote on the subject:

An abnormal MAO-A gene has been shown to be prevalent in serial-killers, mass murderers, paedophiles, sexual sadists and pathological sex offenders.

Sadly, I can't find the source of the quote, nor has the person who posted it got back to me with a source. I can't find any studies or information that study the correlation between an abnormal MAO-A gene and any of those groups.

Can anyone provide links to studies that prove, disprove or even just study such a correlation?

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  • MAO-A breaks down adrenaline, so if MAO-A doesn't work, people are more likely to stay amped after a thrill kill.
    – Cees Timmerman
    Commented Sep 25, 2012 at 13:23

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This is a press hype, pushed up by wrong interpretation of a paper. I have personally researched this incident and put the result in the German Wikipedia. As a good summary and last word on the matter see

Hook, GR: “Warrior genes” and the disease of being Māori MAI Review 2009, 2 http://www.review.mai.ac.nz/index.php/MR/article/viewArticle/222

For the (german) summary see http://de.wikipedia.org/wiki/Monoaminooxidase#MAOA-Varianten_und_menschliches_Verhalten and refs therein.

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    Could you provide a summary of the results of the paper in English? It saves us from link-rot later :)
    – Polynomial
    Commented Jul 18, 2012 at 7:23
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In the wikipedia page you reference is a section called 'Aggression and the "Warrior Gene"'.

This section describes the phenotype you're talking about, and cites this paper (Brunner et al. 1993) that finds loss-of-function point mutations in MAO-A, in a family with 'abnormal' behaviour (defined as "impulsive aggression, arson, attempted rape, and exhibition");

Isolated complete MAOA deficiency in this family is associated with a recognizable behavioral phenotype that includes disturbed regulation of impulsive aggression

This paper has been cited 618 (Scopus) times, and one of the more recent reviews is titled "Genetic determinants of aggression and impulsivity in humans" (Pavlov et al. 2012), and the following is quoted from the abstract;

Genetic predisposition to aggression appears to be deeply affected by the polymorphic genetic variants of the serotoninergic system that influences serotonin levels in the central and peripheral nervous system, biological effects of this hormone, and rate of serotonin production, synaptic release and degradation. Among these variants, functional polymorphisms in the monoamine oxidase A (MAOA) and serotonin transporter (5-HTT) may be of particular importance due to the relationship between these polymorphic variants and anatomical changes in the limbic system of aggressive people. Furthermore, functional variants of MAOA and 5-HTT are capable of mediating the influence of environmental factors on aggression-related traits.

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  • Thanks for the information, but this doesn't address the correlation, if any, between people with MAO-A gene mutation and various serious violent crimes and sex offenses. Also, Brunner Syndrome is one (extremely rare) specific mutation of the MAO-A gene which causes severe MAOA enzyme deficiency, but is not a general case. MAO-A gene mutation encompasses much more than just Brunner Syndrome, and the variety of existing mutation is thought (according to the studies I noted) to be orders of magnitude more prevalent than just Brunner Syndrome.
    – Polynomial
    Commented Jul 16, 2012 at 15:04
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    Brunners is indeed a single study looking at one family with abnormal behaviour (the authors find that all family members have a mutation in MAOA). I went on to search for a recent review paper that cites Brunners study. The review is very interesting (did you read it?) and talks about a LOT of factors that affect aggressive behaviour, of which the serotonin pathway seems very prominent. Therefore genetic variants that affect this pathway will also affect behaviour but calling MAOA the warrior gene is like calling FTO the fat gene. Variants in MAOA may affect behaviour, but do not control it.
    – Luke
    Commented Jul 16, 2012 at 15:17
  • I'm aware of the lack of direct correlation between MAOA and violence, and that it's actually just a contributing factor, but this still doesn't answer the question. I'm looking for studies that aim to find whether there is a correlation between people who have committed serial murder, rape, or are sex offenders, and the presence of mutation in the MAO-A gene. You seem to be looking at the correlation between MAO-A mutations and a predisposition to violence, which is a different matter. A predisposition to violence does not a serial killer make!
    – Polynomial
    Commented Jul 16, 2012 at 15:20
  • In the Bunner case, you see an exaggerated phenotype due to the loss-of-function mutation. At the population level these types of variant will be incredibly rare, so you must use common (present in at least 5% of the population) variants. Common variants do not have big enough effect sizes in outcomes like 'uncontrollable aggression'; the genetic contribution to this phenotype is likely to be comparatively small in comparison to your upbringing. I couldn't find any papers in my searching today that perform a case/control study specifically on murderers/rapists, against population controls.
    – Luke
    Commented Jul 16, 2012 at 15:26
  • Interesting. This is partially anecdotal, but a doctor who was doing research on MAO-A was interviewed as part of the BBC Horizon program I mentioned, and decided to test his family. By total coincidence, it turned out that he had the mutation, and it was very likely, based on character similarities and genetic transfer, that his (by then deceased) father had it too. Empirically, one might say that this is just an incredible coincidence, but more anecdotally one might state that this implies a greater current prevalence than was previously thought.
    – Polynomial
    Commented Jul 16, 2012 at 15:32

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