Genetic Origins of Aggression

The Aggressive Gene in Humans

It is hard to pinpoint a specific gene that causes aggression directly in humans. Aggression results from multiple systems within your body at work, including brain regions such as the amygdala and prefrontal cortex and your serotonin, testosterone, and cortisol ratios.

Research has identified a few genes, however, that play an important role in regulating aggression. One such gene is monoamine oxidase A (MAOA), also infamously known as the warrior gene.

MAOA Gene Mutation

A mutation of the MAOA gene, where there is MAOA deficiency (MAOA-L), can result in aggressive behaviours.

MAOA deficiency (MAOA-L) occurs nearly solely in males. This is because the MAOA gene is located on the X-chromosome. Males only have one X-chromosome, whereas females have two X-chromosome.

Thus, in males, one altered copy of the MAOA gene in the X-chromosome could cause MAOA-L. In females, two altered copies would need to occur (one in each X-chromosome); this is more unlikely to happen, which is why MAOA deficiency affects males much more. Let's explore the MAOA gene traits further.

MAOA Gene Traits

MAOA deficiency can result in notable behavioural issues, many of which link to aggression. Research suggests there is an environmental aspect to these behaviours. So, what are the MAOA gene traits?

Typically, they show:

• Intellectual issues (mild learning disabilities).
• Behavioural issues.
• Aggressive and oftentimes violent outbursts.
• Impulsive behaviours.
• Negative bias in social situations.

Fig. 2 - Research demonstrates how our genes are implicated in aggression.

MAOA Gene: Aggression

The gene thought to influence aggression is known as the MAOA gene. It is responsible for the production of the enzyme monoamine oxidase A, which metabolises the neurotransmitters noradrenaline, serotonin, and dopamine.

The responsibilities and roles these neurotransmitters play varies:

• Serotonin is involved heavily in mood regulation.
• Dopamine is involved in the reward pathways of the brain, affecting motivation.
• Noradrenaline is involved in preparing and arousing the body for action, such as in the fight-or-flight response.

If the MAOA gene is dysfunctional and there is a lack of MAOA activity, these neurotransmitters cannot be broken down. As we discussed briefly above, those with MAOA-L are suggested to have a negative bias for interpreting situations. They will then be more likely to react aggressively, having lower levels of impulse control.

People with the MAOA-L gene display high levels of aggression when provoked compared to those with MAOA-H (normal levels). People with the MAOA-L genotype who suffer maltreatment in childhood generally go on to display aggressive and antisocial behaviours.

However, that is not to say everyone with the MAOA-L gene will go on to become violent, aggressive individuals. Environmental factors have been shown to greatly influence the development of behaviours.

Brunner et al. (1993) studied 28 males from a large Dutch family who were repeatedly involved in impulsive, aggressive, and violent criminal behaviour. The study found that these men had the MAOA-L genotype and had intellectual disabilities, a condition that later became known as Brunner syndrome.

Twin Studies on the Genetic Origins of Aggression

Twin studies present great opportunities for researchers to establish gene and environmental interactions in behaviours.

Monozygotic (identical, MZ) twins share 100% of their genes, and dizygotic (non-identical, DZ) twins share around 50%. Thus, if aggression is influenced by genetics, we expect to find greater similarities in aggressive behaviour between monozygotic twins.

Fig. 3 - There is a degree of heritability in aggressive behaviours.

Adoption Studies on the Genetic Origins of Aggression

Adoption studies about aggression compare concordance rates between an adopted child and their adopted parents, and the adopted child with their biological parents. If they find a positive association between adopted children and their biological parents, this is suggestive of genetic influences, as the children have been raised in another environment.

Animal Research on the Genetic Origins of Aggression

Animal research investigates aggression levels in animals to identify common themes or potential biological explanations. These results are then used to investigate human aggression. They help us form a base knowledge of research and understand animals' behaviour in general.

What are the Strengths of these Studies?

Twin and adoption studies provide support for genetic influences on aggression. While twin studies may find high concordance rates, they can’t investigate nature vs nurture, as the twins are brought up in the same environment.

However, adoption studies look at children who inherit genes from their biological parents but are brought up away from them in a different environment, separating genetics and environment.

What are the Weaknesses of these Studies?

A large number of studies focus on animal research, such as research on mice. However, this research can’t be generalised to humans as we are physiologically different from animals.

There may be issues with how aggressive behaviour is measured in studies. Many studies use questionnaires or hypothetical scenarios, which are subjective and have poor predictive validity. It is difficult to draw conclusions from studies like these.

MAOA Gene: Psychopathy

Psychopathy is associated with genetics. It is accepted that psychopathic individuals inherit genetic makeup that results in altered brain functioning. There is a link between MAOA-L, childhood trauma, and increased risk of psychopathic traits.

In people with MAOA-L genotypes and severe antisocial personality disorder, brain connectivity is different from healthy people.