The ‘Warrior’ Gene

The MAOA gene

Is the single MAOA gene really responsible for violence or has it been over-hyped by the media? If you have seen the following documentary you may be aware of the potential of the role it plays (If you have not seen ‘Are You Good or Evil’ yet, it is a fantastic watch):

However, the uncomfortable fact is that the original ‘warrior gene’ is present in 34% of European males. With many programmes and articles online giving this gene prominence, what do we actually know so far?

The so called ‘warrior’ gene is found only on the X chromosome. Immediately this means that males either express the gene, or do not, since they only inherit one X chromosome, whilst females can be carriers if they only inherit a mutation on one of their X chromosomes. There are actually different ‘levels’ of mutation for the MAOA gene. MAOA, monoamine oxidase A, is an enzyme that is responsible for the breakdown of amines in the brain; serotonin, noradrenaline and dopamine being the main ones. These neurotransmitters play key roles in our mood, appetite, memory, motivation and emotions (such as impulse control). The most common (aka ‘normal’) version of the MAOA gene (4R) has 4-repeats of the same DNA sequence, which leads to high-activity breakdown of neurotransmitters. A faulty version of the MAOA gene is called a ‘low activity variant’, meaning that an individual is unable to breakdown these key neurotransmitters effectively. These supposed risk genes have either 3 (3R) or even 2 (2R) repeats of DNA sequence only. It is important to note, however, that current research indicates that social environment, specifically abuse during childhood, appears to be a necessary requirement for the faulty version of this gene to trigger violent tendencies.

Research into the MAOA gene

The MAOA warrior gene was the first candidate gene to be linked to antisocial behavior, identified in 1993 in a large Dutch family that was notorious for violence. A geneticist called Rod Lea reported that the MAOA-3R mutation was more common in the violent indigenous Maori population of New Zealand than in caucasian males (56% vs 34%). Lea went on to link this gene to a whole host of undesirable traits, such as violence, aggression, risk-taking and gambling/addiction. What has been ignored by the media frenzy surrounding this paper, however, is the fact that the following prevalence rates have been reported by other studies for the 3R genetic variant:

  • Taiwanese males – 61%
  • Chinese males – 56%

Beaver (2013) conducted research into MAOA-3R in America and found that it increased the likelihood of an individual joining a violent gang.

The more extreme, yet very rare, 2R variant of the gene has really drawn the attention of researchers recently. It has been dubbed the ‘extreme warrior gene’. Beaver (2013) found that African American males carrying 2R were more likely to be involved in extreme violence (shooting and stabbing) than African American men with other MAOA variants. Such a finding is controversial, and research in this area socially sensitive, because the gene is not distributed equally throughout different ethnicities. The prevalence rates are 5.5% in African American men, 0.9% in Caucasian men and 0.00067% in Asian men. Research can only thus be carried out in African American men in the US. (source: Add Health sample).

Other recent research into the MAOA gene
Nature-Nurture and the role of gene plasticity

If you have watched the documentary shared above, you will be aware that a diathesis-stress model is very much at play here with the expression of this gene. Those with a faulty version of the MAOA gene only seem to demonstrate violence IF they experience child abuse, or harsh punishments as children. The dual forces of both nature and nurture are required to trigger aggression. In fact, this effect is so strong that a recent review deemed it one of the “the best-supported “observations in the entire literature of psychiatric genetics.”

Choe (2014) looked into the effects of punitive discipline on antisocial behaviour, finding that punitive discipline was only associated with an increase in anti-social behaviour in men with the low activity 3R version of the MAOA gene. There was no relationship found between punitive punishment and antisocial behaviour in men with the 4R version of the gene.

Furthermore, the impact of punishment on antisocial behaviour was linked to age. Those children who received harsh punishments at the ages of 1.5, 2 and 5 years were more likely to develop behaviours, such as violent attitudes and arrest between the ages of 15 and 20.

There is significant hope for individuals carrying the low-activity variants of the MAOA gene. It seems that these genes appear to have high ‘plasticity’ (that is, the ability to be altered by environmental influences), meaning that upbringing will have a greater impact on these individuals – for better or worse. Research has shown that positive parenting leads to improved self-regulation in adolescents with the 2R and 3R alleles, since these alleles appear to have plasticity. In fact, “the more plasticity alleles males (but not females) carried, the more and less self-regulation they manifested under, respectively, supportive and unsupportive parenting conditions.” (Belsky & Beaver, 2011). Parenting therefore plays a big role in the expression of this gene.

The future of research

Most research into the MAOA gene has been observational or based on self-report. Studies into MAOA-2R, which is only really seen significantly in African-American populations, are socially sensitive, due to ethnic differences, however it will be interesting to see whether this gene affords a genetic predisposition only, or acts independently of environmental factors. It is difficult to untangle the effects of genes and the environment, to determine which is more important in predicting violent behaviour, however, current research makes it quite clear that gene plasticity may play an important role, with male children carrying a low-activity version of the MAOA gene being more significantly influenced by environmental factors in their upbringing.


Beaver K, Barnes J, Boutwell B. The 2-repeat allele of the MAOA gene confers an increased risk for shooting and stabbing behaviors. Psychiatr Q. 2013a. Dec 11.

Belsky J, Beaver KM (May 2011). Cumulative-genetic plasticity, parenting and adolescent self-regulation. Journal of Child Psychology and Psychiatry 52 (5): 619–626.

Choe D, Shaw D, Hyde L, Forbes E. Interactions between monoamine oxidase A and punitive discipline in African American and Caucasian men’s antisocial behavior. Clinical Psychological Science.2014. March 14. doi:10.1177/2167702613518046

Lea R, Chambers G. Monoamine oxidase, addiction, and the “warrior” gene hypothesis. N Z Med J. 2007.

Oubré (2014). The extreme warrior gene a reality check. Retrieved from:

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