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There is much debate about whether aggression is a learned or innate behaviour. Emotional and control disorders are linked to disturbances in the brain’s normal activity, due to altered gene expression and chemical imbalances. Genes don’t affect aggression directly, but they affect the production of hormones and neurotransmitters, which then affects aggression. Interestingly, genetic influences explain up to 50% of the total variance in aggressive behaviour.
A person’s genotype (their genetic makeup) and environmental factors such as stress and diet influence how the brain functions. Abnormal versions of the monoamine oxidase A (MAOA) gene, also known as the warrior gene, often result in aggression.
Three strands of DNA, pixabay.com.
Monozygotic (identical, MZ) twins share 100% of their genes, dizygotic (non-identical, DZ) twins share around 50%. Thus, if aggression is influenced by genetics we would expect to find greater similarities in aggressive behaviour between monozygotic twins.
Coccaro et al. (1997) found that aggressive behaviour concordance rates for twins (proportion as a percentage where both twins have the same behaviour or characteristic) are:
Physical aggression: MZ=50% and DZ=19%
Verbal aggression: MZ=28% and DZ=7%
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.
A meta-analysis by Rhee and Waldman (2002) of adoption studies found that genetic influences account for around 41% of the variance in aggressive behaviour.
Twin babies in blue and pink blankets, flaticon.com/freepik.
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 to form a base knowledge of research and to understand the behaviour of animals 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. This research 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.
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.
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.
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. If the MAOA gene is dysfunctional, these neurotransmitters can’t be broken down.
If there is too much noradrenaline:
Noradrenaline prepares and arouses the body for action. Too much noradrenaline leads to hypersensitivity in the fight-or-flight response, as the body always believes it is in a stressful situation.
People then overreact to perceived threats when there may not be any.
If there is too little serotonin:
Although MAOA dysfunction leads to too much serotonin, many aggressive behaviours are associated with too little serotonin which leads to reduced control over impulsive behaviour.
If there is too much dopamine:
This increases the probability of having feelings of reward after an aggressive act.
Low levels of MAOA (allele MAOA-L) are associated with aggression. People with MAOA-L gene display high levels of aggression when provoked. However, just having the gene doesn’t mean a person will be aggressive. Only people with the MAOA-L genotype who suffer maltreatment in childhood generally go on to display aggressive behaviour.
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.
Brunner et al. (1993) studied 28 males from a large Dutch family who were repeatedly involved in impulsive aggressive violent criminal behaviour. The study found that these men had the MAOA-L genotype and intellectual disability, a condition that later became known as Brunner syndrome.
Psychopathy is primarily due to 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.
Emotional and control disorders are linked to disturbances in the brain’s normal activity due to altered gene expression and chemical imbalances.
Genetic influences explain up to 50% of the total variance in aggressive behaviour.
Twin and adoption studies provide support for genetic influences on aggression. However, a lot of genetic research is on animals, which can’t be generalised to humans.
The MAOA gene produces the protein monoamine oxidase which metabolises the neurotransmitters noradrenaline, serotonin, and dopamine. A mutation of the MAOA gene causing low levels of MAOA (allele MAOA-L), leads to aggression.
People with the MAOA-L genotype who suffer maltreatment in childhood generally go on to display aggressive behaviour.
There is a link between MAOA-L, childhood trauma, and increased risk of psychopathic traits.
The three types of aggression are reactive-expressive aggression, reactive-inexpressive aggression, and proactive-relational aggression. Reactive-expressive aggression is characterised by physical and verbal behaviour (such as screaming) that is intended to hurt someone. Reactive-inexpressive aggression is characterised by hostility. Proactive-relational aggression is characterised by behaviour that is meant to harm the social relationships of a person or group, such as teasing or intimidation. It is a type of aggression that is used to obtain a goal or benefit.
Genetic influences account for up to 50% of aggressive behaviour. Environmental factors account for the other 50%. Thus, a person's genotype and environment both shape their brain function and behaviour.
Genetics can play a part. However, learned behaviour is also a factor. This may be due to modelling, which is when children observe a behaviour and then copy it, or due to reinforcement. For example, if a child gets what they want by lashing out in anger.
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