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Genetic Origins of Aggression

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Genetic Origins of Aggression

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.

Genetic Origins of Aggression DNA strands StudySmarterThree strands of DNA, pixabay.com.

Twin studies on the genetic origins of aggression

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 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.

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.

Genetic Origins of Aggression Twin babies illustration StudySmarterTwin babies in blue and pink blankets, flaticon.com/freepik.

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 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.

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.

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.

Evidence of the MAOA gene

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.

MAOA gene and psychopathy

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.


Genetic Origins of Aggression - Key takeaways

  • 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.

Frequently Asked Questions about Genetic Origins of Aggression

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.

Final Genetic Origins of Aggression Quiz

Question

How do genes affect aggression?


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Answer

Genes don’t affect aggression directly, but they affect the production of hormones and neurotransmitters, which then affects aggression.


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Question

What two factors influence brain function and behaviour?


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Answer

A person’s genotype and environmental factors.


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Question

How much of the total variance of aggressive behaviour can be explained by genetic influences?


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Answer

50%

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Question

Why are twin studies useful in studying the genetic origins of aggression?


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Answer

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


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What is the strength of adoption studies over twin studies?


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Answer

In twin studies, the effects of the environment can’t be ruled out, 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.


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Why are animal studies a weakness in explaining the genetic factors of aggression?


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Answer

This research cannot be generalised to humans as we are physiologically different from animals.


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What mutation of the MAOA gene leads to aggressive behaviour?


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Answer

MAOA-L

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How does the MAOA gene lead to aggressive behaviour?


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Answer

The MAOA gene is responsible for the production of a protein which metabolises the neurotransmitters noradrenaline, serotonin and dopamine. If the MAOA gene is dysfunctional, these neurotransmitters cannot be broken down.


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Question

What happens if there is too much noradrenaline in the body?


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Answer

There is hypersensitivity in the fight-or-flight response, which leads to overreaction to perceived threats when there may not be any.


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What happens if there is too little serotonin in the body?


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Answer

A person has reduced control over their impulsive behaviour.


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What happens if there is too much dopamine in the body?


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Answer

It increases the probability of having feelings of reward after an aggressive act.


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Question

What is serotonin?

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Answer

It is a neurotransmitter.

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If a person has normal serotonin levels, what can they usually do?

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Answer

Normal levels of serotonin in the orbitofrontal cortex (OFC) correlate with greater self-control levels. Decreased levels of serotonin usually mean a person acts more impulsively.

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What are the critical components of a functioning serotonin system?

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Serotonin synthesis, serotonin transportation/uptake, and serotonin receptors/degradation.

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Do genes have direct control over behaviours?

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No. Genes may not directly affect behaviour, but how genes act on the regulators, i.e., the neurotransmitters that influence behaviours, are how they affect behaviours.

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What amino acid produces serotonin?

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Answer

The amino acid tryptophan.

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What important enzyme is part of tryptophan?

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Answer

Tryptophan hydroxylase (TPH).

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What breaks down serotonin?

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Answer

Monoamine oxidase A and B (MAOA and MAOB).

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What did Cases et al. (1995) find in their study?

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Answer

For mice who had their MAOA gene deleted, serotonin concentrations increased ninefold, as MAOA could no longer break down the serotonin in the mice’s brains. Adult male mice showed increases in aggressive behaviours.

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What did Walther et al. (2003) find in their study?

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Answer

The TPH gene was not the only one in the genome that affects serotonin synthesis. TPH affects serotonin levels in the blood, periphery tissues, and pineal gland. TPH2 is what affects serotonin levels in the brain.

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What did Kulikov et al. (2005) find in their study?

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In this study on mice, the TPH2 gene was genetically altered (knockout mice) to see its effects on serotonin and aggression. They changed a particular section of the allele from C to G on the gene. Mice who had the original C gene had higher levels of 5-HT. These mice were more prone to attacking other mice if they intruded on their territory.

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Question

What did Holmes et al. (2002) find in their study?

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Answer

In this study, the serotonin transporter (5-HTT, or SERT) was knocked out, and they assessed the aggression level of these mice in cages. The SERT deficient mice were less inclined or slower overall with attacking the intruder than the controls.

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Question

What did Brunner et al. (1993) find in their study?

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Answer

Brunner et al. (1993) assessed a Dutch family and found a genetic issue causing a mutation in the structure of the MAOA gene. 14 affected men had complex behavioural issues (related explicitly to aggression) and were affected by a syndrome of borderline mental retardation.

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Question

What did Williams et al. (2003) find in their study?

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Answer

Women with specific genotypes had higher levels of 5HIAA (a byproduct of the breakdown of serotonin) than men. Similarly, African Americans with particular genotypes had higher levels of 5HIAA, too, when compared to Caucasians.

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Question

What do we mean when we say knockout mice?

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In this case, it means the gene in these mice has been altered/deleted or knocked out.

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Is the evidence for serotonin causing aggression causal?

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No. It is causal in its relation to mood regulation, but not for aggression. It is only correlational.

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Question

What is the MAOA gene?

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Answer

We refer to the monoamine oxidase A gene when talking about the MAOA gene.

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What does the MAOA gene do?

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The MAOA gene codes for producing the enzyme MAOs (monoamine oxidases), involved in breaking down neurotransmitters in the synapses between neurones.

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Question

Where is the MAOA gene found?

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The gene is found on the X chromosome.

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Why would dysfunction of the MAOA gene affect a person’s mood?

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Answer

The gene codes for enzymes that break down neurotransmitters such as serotonin. If the gene cannot do this properly, these neurotransmitters are left in the synaptic cleft for longer and affect a person’s mood.

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Question

Who re-introduced the Warrior Gene and gave it the ethically questionable connotations?

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Answer

Dr. Rod Lea.

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Question

What traits are associated with the warrior gene?

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Answer

Aggressive behaviours, risk-taking, addiction issues, and psychiatric disorders are associated with the warrior gene.

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Question

How many Mãori men had the MAOA gene variant compared to Caucasian men?

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Answer

56% of the Mãori men had this MAO-30bp-rpt allele, almost double that of Caucasian men analysed in a different study.

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Question

What did Brunner et al. (1993) find in their study?

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Answer

There was a point mutation in the MAOA structural gene (specifically the eighth axon). This changed how this gene coded for the enzyme production, which caused issues with the breakdown of neurotransmitters.

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Question

What did the men in Brunner et al. (1993) show in their behaviours?

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The behaviours of the men in Brunner et al. (1993) consisted of impulsive aggression, arson, and attempted rape.

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What did Lea state the Mãori men were more likely to be like?

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Answer

‘Obviously, this means they are going to be more aggressive and violent and more likely to get involved in risk-taking behaviour like gambling.’

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What reportedly is the cause of the high occurrence of the MAOA (warrior) gene in the Mãori men?

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Lea suggested this was due to the nature of the Mãori men’s past; they had to engage in many risk-taking behaviours, such as migration and fighting for survival, which has led to aggressive behaviours in the present, modern-day, and a genetic bottleneck.

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Question

Give a strength of the genetic research on the MAOA gene.

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Answer

The MAOA gene is fundamentally linked to mood due to producing enzymes that deal with neurotransmitters. Mood and behaviours will also be affected if the gene is affected. This suggests a genetic component to behaviour and moods, particularly aggression, and validates the genetic research on the MAOA gene.

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Question

What did Caspi et al. (2002) find in their study?

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Answer

They found that 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 after suffering maltreatment.

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Give one weakness of the genetic research on the MAOA gene.

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Answer

McDermott et al. (2009) found in their study that the MAOA gene is not explicitly tied to aggression, even in low provocation conditions. Instead, it predicts aggressive behaviours in high provocation situations.

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Question

Is the ‘warrior gene’ unethical?

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Answer

Yes, the ‘warrior gene’ is unethical as it has racial overtones that unfairly describe a race of people as aggressive due to their genetic makeup.

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Question

What is testosterone?

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Answer

It is an androgen and anabolic steroid.

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What is the neuroendocrine system?

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A neuroendocrine system is a group of neurones, glands, and other tissues that regulate homeostasis (this is the normal, steady, and optimal state of the body).

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Why do brain centres arouse the neuroendocrine system during aggressive behaviours?

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Answer

This metabolic arousal results in the expression of aggression through mobilising the body’s muscles.

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Question

Where is testosterone produced?

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Answer

The Leydig cells in the testes produce testosterone (remember, testosterone is produced in the gonads for both sexes).

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What gene is involved in the production of testosterone in Leydig cells?

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The NR2F2 gene.

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What did Bogaert et al. (2008) find in their study?

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Answer

They found the sex steroid concentrations (testosterone, etc.) and the body composition factor had significant heritability, with testosterone being the highest. They concluded that these two factors are under strong genetic control.

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What did Meikle et al. (1986) find in their study?

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Answer

In twins whose blood levels (especially testosterone) were measured, the familial influence was more significant in MZ twins than in DZ twins, which was true for all measurements except SHBG.

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What did Brunner et al. (1993) prove in their study?

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Answer

They proved that specific genes influence aggressive behaviours by identifying a mutation in the MAOA gene.

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Question

What did Albert et al. (1993) summarise in their review?

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Answer

Although aggression in humans does have a biological root in defensive aggression, it’s not dependent on hormones such as testosterone.

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