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Genetic factors in aggression

Genetic factors in aggression

Human aggression-related traits have a complex background that is not only regulated by non-genetic effects (such as neural and hormonal influences) but also by genetic factors (one’s genetic makeup\).

(Introduction and) studies to prove genetic and hereditary foundations for aggression

1) Twin studies: In this type of study, researchers compare the degree of similarity for a particular trait, in this case, aggression, between sets of monozygotic (identical) twins and dizygotic (non-identical) twins. The initial hypothesis was that if a set of monozygotic twins were similarly aggressive in their behaviour, then this was due to their genetic makeup rather than their environment. This was supported by the fact that both types of twins shared the same environment, but monozygotic twins were more similar in their behaviour.

2) Adoption studies: Hutchings and Mednick 1975

A study of over 14000 adoptions in Denmark found that a significant number of adopted boys with criminal records had biological parents with convictions for criminal violence, especially fathers, providing evidence for genetic makeup affecting aggression.


- 36.2% of the boys were likely to have a criminal record if both biological and adoptive fathers had a criminal record

- 21.4% if only biological

- 11.5% if only adoptive

- 10.5% if neither father had a criminal record

A certain gene for aggression?

Despite not being able to identify a specific and individual gene in humans for aggression, researchers have been able to associate a gene responsible for producing the enzyme monoamine oxide (MAOA) with aggressive behaviour. This enzyme is responsible for breaking down several neurotransmitters, including serotonin, and is also responsible for regulating serotonin levels in the brain (low levels of serotonin are linked to impulsive and aggressive conducts).

Evidence for the role of MAOA in aggression

· Data from a 2007 study showed that individuals with the MAOA gene are hypersensitive, being easily affected by negative experiences and emotions, thus reacting more aggressively when defending themselves.

· In 1980s, male members of a large Dutch kindred were studied. It was noticed that many of the male members who displayed abnormally violent behaviour and acted in a particularly aggressive manner (with a large proportion of them having been involved in serious crimes of violence such as arson and rape) were all recognised to have low levels of MAOA in their bodies, whereas typical male members within the family did not carry this mutation.

MAOA variants


Caspi et al 2002

In a study done on 500 male children, researchers found a variant of the gene associated with high levels of MAOA (MAOA-H) and a variant associated with low levels (MAOA-L). Maltreated children with a genotype with high levels of MAOA expression were found to be less likely to develop antisocial problems, whereas maltreated children with the MAOA-L variant were more likely to exhibit antisocial behaviour.

The warrior gene

The “warrior gene” encapsulates particular variations of the x-chromosome gene which produce MAOA, the enzyme used to affect the neurotransmitters serotonin, norepinephrine, and dopamine. It’s also known as MAOA- L.

The “warrior gene” occurs much more frequently in populations with a history of warfare, with around 66.66% of people in those populations having the gene. In contrast, 33.33% of the population in western countries have a low-activity version of this gene.


The main MAOA warrior gene variant (the 3R variant) is found in

· 56% of Maori males (New Zealand indigenous people)

· 58% African American males

· 34% European males

· 61% of Taiwanese males

· 56% of Chinese males

Note: The reason these results are given only for men and not for women is due to the fact that the majority of research done on MAOA is focused predominantly on men. This could be because women have two X chromosomes, therefore decreasing the probability of inheriting an active version of a MAOA mutation.



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