Is aggression a learned or innate behaviour? We know from research that emotional and control disorders are linked to disturbances in the brain’s normal activity, possibly due to altered gene expression and chemical imbalances. Genes do not affect aggression directly, but they affect the production of hormones and neurotransmitters, which then affect aggression. Let's explore the genetic origins of aggression.
- First, we will look at the potential aggressive genes in humans, focusing specifically on monoamine oxidase A (MAOA).
- We will learn about how a mutation in the MAOA gene causes aggression.
- Then we will look at some studies on the genetic origins of aggression both in people and animals.
- Finally, we will look at the link between the MAOA gene and psychopathy.
Fig. 1 - Genes play a role in the development of our behaviours.
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.
A person’s genotype (genetic makeup) and environmental factors such as stress and diet influence how the brain functions.
MAOA Gene Mutation
A mutation of the MAOA gene, where there is MAOA deficiency (MAOA-L), can result in aggressive behaviours.
MAOA gene encodes an enzyme that helps break down neurotransmitters such as serotonin, noradrenaline and dopamine.
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.
Frazzetto et al. (2007) found a link between low MAOA activity (allele MAOA-L) and aggressive behaviour, but only in people who had experienced childhood sexual or physical abuse.
People with the MAOA-L gene who didn’t experience childhood abuse didn’t display any aggressive behaviours.
Many suggest the link between MAOA and the development of aggression is neurodevelopmental, as a result.
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.
Coccaro et al. (1997) investigated the influences of genetics and environment on aggressive and irritable behaviour in male twins who were on the Vietnam Era Twin (VET) Registry. This registry is of American veteran male twins who were born between 1939 and 1955 and served in the United States armed forces between 1964 and 1975.
The average age of the participants was 44 years old. Participants were mailed the 'Motor Aggression' subscales of the Buss-Durkee Hostility Inventory (BDHI) to fill out. Data from 182 MZ twins and 118 DZ twins were analysed. The researchers found that aggressive behaviour concordance rates for twins (correlation) are:
| Scale | MZ | DZ |
| Direct Assault | 0.50 | 0.19 |
| Indirect Assault | 0.42 | 0.02 |
| Verbal Aggression | 0.28 | 0.07 |
| Irritability | 0.39 | -0.06 |
We classify indirect assault as roundabout and indirect aggressive methods such as temper tantrums or slanderous gossip. Verbal aggression refers to behaviour such as arguing, shouting, and screaming, and irritability refers to feeling like their temper can go off at the slightest annoyance.
They suggest that impulsive aggression can be heritable in men.
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.
Rhee and Waldman (2002) conducted a meta-analysis of 51 twin and adoption studies to investigate the influence of genetics and the environment on anti-social behaviour.
The adoption studies were of two types:
- Compared adoptive children and their adoptive parents and also biological parents.
- Compared adoptive siblings and biological siblings.
They found that genetic influences correlated for 0.41 of the variance in aggressive behaviour. Meanwhile, environmental influences accounted for 0.16.
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.
Lagerspetz (1979) bred 25 generations of mice. In each generation, she bred the most aggressive mice together and the least aggressive mice together.
This resulted in two groups of very different mice: one group of super-aggressive mice, and the other of super-docile mice.
The study supports the role of genes in aggression as it showed that the offspring of two mice with the aggressive gene is markedly more likely to be aggressive.
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.
Tiihonen et al. (2020) found that psychopaths have differences in 6 genes compared with healthy controls. Dramatic upregulation of RPL10P9, ZNF132 (this results in there being too many of these genes) and in astrocytes RPL10P9 and MT-RNR2, alongside downregulation of CDH5 and OPRD1 (this results in a lack of these genes).
Genetic Origins of Aggression - Key takeaways
Emotional and control disorders are linked to brain activity disturbances due to altered gene expression and chemical imbalances.
Genetic influences explain variance in aggressive behaviours. Studies suggest there is an interaction between our genes and the environment in the development of aggression.
Twin and adoption studies provide support for genetic influences on aggression. However, a lot of genetic research is on animals, which cannot be generalised to humans.
The MAOA gene produces the protein monoamine oxidase, which metabolises the neurotransmitters noradrenaline, serotonin, and dopamine.
People with the MAOA-L genotype who suffer maltreatment in childhood generally go on to display aggressive behaviour. Studies suggest there is a neurodevelopmental link between MAOA and aggressive, antisocial behaviours in adulthood.
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