Should people with a genetic predisposition to aggression be punished for violence? This question became particularly important in the court case of Abdelmalek Bayout, an Algerian male, who was sentenced for stabbing a man to death in Italy in 2007. His initial sentence was reduced by a judge because Abdelmalek had the Warrior Gene, which has been linked to aggression.

So, is there any scientific basis for the Warrior Gene to be used as a get-out-of-jail-free card?

• First, we'll look at the warrior gene definition.
• Next, we'll introduce the warrior gene theory of aggression.
• Then, we'll consider the origins and history of the Maori warrior gene.

• Moving along, we'll briefly explore the case of the warrior gene in females.

• Finally, we'll evaluate the MAOA Warrior Gene theory of aggression.

Fig. 1 - The Warrior Gene theory of aggression proposes that genetic factors can predispose us to aggression. Can our genes determine our actions?

Warrior Gene Definition

The warrior gene, also called the MAOA gene, codes for an enzyme that is crucial for breaking down monoamines, including serotonin.

Serotonin is one of the primary neurotransmitters broken down by MAOA, although dopamine and norepinephrine are also affected.

Many refer to the MAOA gene as the ‘Warrior Gene’ due to its ties with aggression. This is not to say that these ties are factual and proven, and we will assess the studies to determine the validity of their findings.

How does the MAOA Warrior Gene Affect Mood?

Neurotransmitters are fundamental in regulating moods and subsequently behaviours. As MAOs are enzymes that break down these neurotransmitters, any issues with the MAOA gene and its ability to produce these enzymes would affect a person’s mood.

If neurotransmitters are left in the synaptic cleft, it can cause many issues. The neurotransmitter effects are ultimately prolonged, resulting in continued activation of the neurones involved.

Warrior Gene Theory of Aggression

As MAOA is involved in the production of enzymes that break down neurotransmitters, issues with this gene can result in mood disorders, as seen in the case of Brunner et al. (1993), where Brunner Syndrome was established.

In this study, 28 males in a Dutch family were investigated, as they were showing signs of abnormal behaviour and borderline mental retardation.

• Researchers analysed the participants' urine over 24 hours and found a deficiency in the MAOA enzyme activity.

• In 5 affected males, further investigation revealed a point mutation in the MAOA structural gene (specifically the eighth axon). This changed how this gene coded for enzyme production, which caused issues with the breakdown of neurotransmitters.

If the serotonin cannot be broken down properly, the level of serotonin increases, affecting mood and behaviour. This finding suggests that the MAOA gene mutation is linked to abnormal, aggressive behaviours.

Māori Warrior Gene

The MAOA Warrior Gene was the subject of a New Zealand study conducted by Dr Rod Lea in 2006, which found the ‘warrior gene’ in Māori men, explaining their aggressive behaviours and lifestyles (Lea & Chambers, 2007).

Lea stated that several negative behaviours are associated with a certain variation of the warrior gene.

When genotyping 46 unrelated Māori men, researchers found the following:

• 56% of the Māorimen had this variation of the MAOA gene, almost double that of Caucasian men analysed in a different study.

Further identification of different polymorphisms of the MAOA gene revealed that:

• 70% of Māori men compared to 40% of non-Māori men had this variation of the MAOA gene.

Fig. 2 - Lea & Chambers (2007) found a higher prevalence of the Warrior Gene in Māori men compared to Caucasians¹.

Lea reportedly stated to the media (Wellington: The Dominion Post, 2006):

This statement is ethically questionable and raises many questions, namely, is it fair to describe all men with this gene as aggressive and violent?

Lea suggested this was due to the nature of the Māori men’s past. They had to engage in many risk-taking behaviours, such as migration and fighting for survival, which has then led to aggressive behaviours in the present, modern-day, and a genetic bottleneck. The study suggests that this genetic variation might have evolved due to natural selection, and continued to be present in Māori men.

According to Lea, the gene was dubbed the Warrior Gene due to the culture of the Māori men, who value their ‘warrior’ traditions, which remain a part of their culture today.

Warrior Gene in Females

The Warrior gene is found on the X chromosome, which means that it is sex-linked. Due to its location, only males inherit a single copy of this gene and are affected by it. However, females can still be carriers of this gene.

Evaluation of the MAOA Warrior Gene Theory of Aggression

First, let's explore the strengths of the warrior gene theory.

• Research in favour of the theory: Brunner et al. (1993) found that the presence of a mutation in the MAOA gene was associated with aggressive and violent behaviours, this suggests that the MAOA gene can lead to aggressive behaviours if defective.

• Caspi et al. (2002) assessed a large sample of male children from birth to adulthood. The study wanted to investigate why some maltreated children develop antisocial behaviour, whilst others don't.

  • They found the MAOA gene was important in moderating the effect of maltreatment.

  • If children had a genotype that expressed high levels of MAOA, they were less likely to develop antisocial behaviours.

  • This suggests genotypes can moderate children’s sensitivity to maltreatment and the development of aggressive behaviours.

• Associations between the gene and behaviour regulation: As mentioned in the studies above, the MAOA gene is fundamentally linked to mood due to the need to produce enzymes that deal with neurotransmitters. If the gene is affected, it stands to reason that mood and behaviours will be affected, too.

Now, let's explore the weaknesses of the warrior gene theory.

• Aggression occurs only when provoked: In the study of McDermott et al. (2009) subjects were paid to punish people they believed had taken money from them.

  • People with low activity MAOA genes only behaved aggressively in the lab when provoked.

  • It suggests that the MAOA gene is not explicitly tied to aggression, even in low provocation conditions, but instead, it predicts aggressive behaviours in high provocation situations.

  • This finding suggests that the MAOA gene is only associated with aggression if the subject is provoked.

• Reductionist: The suggestion that a gene is responsible for violent or aggressive behaviours reduces all the causes of human behaviour down to biology. It ignores the environmental factors that may significantly influence a person’s choices and behaviours. It oversimplifies the nature of behaviour.

• Deterministic: If a gene controls human behaviour outright, with no room for a person’s free will or choices to decide what they want to do, it can create many issues for society. If a person is more inclined to be violent purely because they have a gene for it, is it fair to treat them similarly to everyone else? Should they be prosecuted for violent behaviour when they are helpless but follow their biological urges?

• Merriman and Cameron (2007): In their review of the 2006 study, whilst they agree that there is an association between the genetic variant of MAOA and antisocial behaviours in Caucasians, the study has no direct evidence to suggest there is an association for Māori men. Overall, they criticise the warrior gene study, suggesting the conclusions were based on ‘science with insufficient investigative rigour’ in applying new literature and understanding older, relevant literature.

• Ethical issues: The term warrior gene is ethically problematic, as it reduces a person’s nature to their genetic predispositions, ignoring other aspects of their character and their overall free will to make moral choices. It has connotations that are not fair to place on a whole race of people.