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Genetics of Aggression

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Genetics of Aggression

Aggression can be explained psychologically and biologically. One of the main biological explanations of aggression is genetics. The proposition is that our genes control our behaviour and if someone is aggressive, then that is due to behaviours that they have inherited through their genes.

Aggression in psychology refers to behaviours that could harm yourself, and others, or could affect objects in the environment. This harm can be physical or psychological.

Genetics of Aggression Genetic explanation, aggressive behaviour,  StudySmarterAggressive Behaviour, freepik.com

Genetic Explanation of Aggression

Genes are inherited from our parents and are innate. They consist of small sections of DNA, which produce instructions for both your genotype and phenotype. Genes can affect the psychological features of an organism, for instance, some psychologists have suggested there is a connection between intelligence and your genes (although most research points to intelligence being a polygenic trait).

Another example can be seen in schizophrenia, which was suggested to be due to a single 'schizogene', but more recent research has identified that it too is a polygenic disorder.

What we can say is that genes don't directly affect aggression, but they do alter certain neurotransmitters and hormones levels, which can cause chemical imbalances and other gene expressions, which in turn disturb brain activity and may cause aggression.

One example is the monoamine oxidase gene (MAOA, a.k.a. the 'Warrior' gene). MAOA is a gene that codes for an enzyme involved in the breakdown of certain neurotransmitters, namely serotonin, which is a key component of mood regulation.

Dysfunction of the MAOA gene can result in high levels of aggression and impulsivity, as seen in the Dutch study by Brunner (1996). Affected males showed issues with both their mental capabilities and an increase in violent behaviours and acts.

The different neurotransmitters and hormones involved in aggression will be covered more in-depth in separate sections, but here we will look at basic evidence of the genetic influence on aggression.

Genetics of Aggression Genetic explanation of aggression, Genes, StudySmarterGenes, flaticon.com

Basic Evidence of Genetic Influences on Aggression

There are different types of genetic influences on aggressive behaviour, and extensive research has been conducted (both the basis of the theories and research support for it) on each type, which provides evidence for the genetic basis of aggression.

Role of Genetic Factors in Aggressive Behaviour

The main things we will cover in this section are:

  1. Twin studies

  2. Adoption studies

  3. Gene-Environment Interaction studies.

Twin Studies

Studies have been conducted using different types of twins to investigate the level of influence that genes have on aggressive behaviours.

There are two different types of twins:

  1. Monozygotic (Mz) Twins - These are twins that are identical and share 100% of their genes.

  2. Dizygotic (Dz) Twins - These are non-identical twins who only share 50% of their genes on average.

Genetics of Aggression Genetic explanation of aggression  StudySmarterMz Twins, freepik.com

The reason that twins have been studied is that by comparing both types of twins we can see to what extent a behaviour is influenced by genes. One of the major benefits of studying twins is the ability to identify how much the variability in the results are a result of their genes and the environment. As MZ twins share 100% of their DNA, if something is purely a result of their genes, there would a 100% concordance rate in the twins.

The American Psychological Association defines concordance rase as:

The percentage of pairs of twins or other blood relatives who exhibit a particular trait or disorder.

MZ twins are usually expected to have higher concordance rates than DZ twins. A higher concordance rate typically means there is a genetic component to the thing being tested.

MZ twins would only differ in their environment, and the hope is that if they were raised together, these factors should also be mostly similar.

Coccaro et al. (1997) studied both MZ and DZ twins and hypothesised that MZ twins would have greater similarities (higher correlation) than DZ twins in aggressive behaviour (if aggression was indeed influenced by genes).

They found that for aggressive behaviour described as direct physical assault, MZ twins had higher concordance rates (50%) than DZ twins (19%). For aggressive behaviours described as verbal assault, MZ twins had higher concordance rates (28%) than DZ twins (7%).

These results show that there is a significant influence on genes of aggression.

Genetics of Aggression Genetic explanation of aggression  StudySmarterDz Twins, freepik.com

Adoption Studies

There has been a lot of research conducted investigating adopted children and comparing them to both their biological and adoptive parents. This is done to see whether behaviours are influenced by their genes or environment since the child will share genes with the biological parents, but their environment with the adoptive parents.

Rhee and Waldman (2002) conducted a meta-analysis of 51 twin and adoption studies that looked at direct aggression and antisocial behaviour (which has aggressive behaviour as an important feature).

They found that genetics could explain up to 41% of the variance in aggression, which is similar to the findings and conclusion of the twin studies.

Genetics of Aggression Genetic explanation of aggression, adoption,  StudySmarterAdoption, freepik.com

Gene-Environment Interactions

From the last two types of studies, we can see that while genetics may influence aggression, they aren't the only influence, and aggression isn't determined by them, otherwise, we'd find concordance rates close to 100% for Mz twins and 50% for Dz twins.

Our environment can also be a factor and influence aggression. It has been found that the MAOA-L gene only has an effect on adult aggression when they've experienced trauma early on in life.

Frazzetto et al. (2007) investigated the link between early life trauma and aggressive behaviours in adult life in men. They found an association between the MAOA-L variant and antisocial aggression, as it is a low activity variant. This suggests the low activity of the MAOA gene in modulating neurotransmitter breakdown affected the expression of aggression.

But this was only the case in men who had experienced significant trauma before they turned 15 years old (like physical or sexual abuse). Those who didn't have such experiences didn't have particularly high levels of aggressive behaviour, even if they had the MAOA-L variant, which highlights the importance of environmental factors.

Genetic Predisposition to Aggression

Considering the above studies, it show that genes don't have a sole influence on aggressive behaviour, but rather just create a 'predisposition' (susceptibility) for aggressive behaviour, which then manifests when there is an interaction with the appropriate environmental factors.

This provides strong supporting evidence for the Gene-Environment Interactions explanation (sometimes referred to as the Diathesis-Stress model).

Evaluation of Genetic Factors in Aggression

Let's consider the strengths and weaknesses of the evidence of genetic influences in aggression.

Research Support

Strength: There has been further research support that shows the genetic influence on aggression in the form of twin studies. Christiansen (1977) went through Danish police records and found that there was a stronger correlation between MZ twins than DZ twins when it came to criminality. However, it can be argued that criminality is not the same as aggression.

Validity

Strength: Because MZ and DZ twins and the proportion of genes they share (100% and 50% respectively) are naturally occurring variables, in that researchers aren't manipulating anything, it means there's little to no researcher bias.

Weakness: Twin Studies may lack validity because while twins do share genes, they also share environments too since they grow up together (in most cases). It could be argued that if MZ and DZ twins both share environments in the same way (equal environments assumption) but Mz twins still have higher concordance rates, then that shows the influence of genes, not the environment. However, that isn't entirely true. MZ twins share their environment more than DZ twins. Mz twins may be treated in a much more similar way than DZ twins, especially by their parents. Therefore, the higher concordance rates in MZ twins may very well be because of the environment rather than genes.

Generalisability

Weakness: Because adoption studies only look at children who have been adopted, it may not be valid to generalise the findings to other, nonadopted children. The reason they were adopted (e.g. abandoned or unwanted by the biological parents, or death of biological parents) could potentially act as an early traumatic experience for them, which has been linked to the development of aggression in studies mentioned above. Twin studies will also only apply to twins, which isn't representative of other children. There aren't that many twins either (1.5% of UK births, and MZ twins are only 0.5%).

Reliability

Weakness: In adoption studies, there is an opportunity sample. Also, not all adoption cases are the same, so there are many extraneous variables, e.g:

  • Children who are adopted due to less extreme situations (immigration issues rather than parent death).

  • Children who have spent time as orphans before getting adopted

  • Children who have been fostered by other families before getting adopted.

These problems have been 'countered' by using very large sample sizes so these anomalies are averaged out, but this makes it very time consuming, expensive and difficult.

Ethics

Strengths: In adoption studies, the children can't consent to partake in the study, however, they're kept anonymous and presumptive consent is taken from the adoptive (and if possible, biological) parents. This is the same for twin studies.

Strengths: With twin studies, many researchers advertise for adult twins to partake in their study, or even go through hospital records and contact twins that were born there to ask them to take part.

Weakness: The studies could cause psychological/emotional harm by highlighting the differences between the adopted child and their adoptive siblings and parents, which could put a rift in their relationship with each other. They should all be carefully debriefed to avoid such things as much as possible. They should also not be made to feel unusual or different, and the BPS warns against not making subjects have any lowered self-confidence. This can be applied to twins being studied too.

Reductionist and Deterministic:

Weakness: When using genetic research, it can raise issues of the theories being reductionist in that they ignore important environmental factors which have been linked to the expression of genes. By identifying a gene association in aggression, we can ignore the situation surrounding the behaviours, which may have a stronger impact than the gene alone.

The theory of genes influencing aggression is also deterministic, as it implies that aggression is a biological behaviour that cannot be ignored, we are predetermined to behave aggressively and free will is dismissed. This has multiple implications for society. For instance, if someone is genetically predisposed to being aggressive and committing aggressive and violent acts, can we hold them responsible for this, and then judge them as a criminal? Is this fair, when technically they can argue it is not their fault?


Genetics of Aggression - Key takeaways

  • Aggression in psychology refers to behaviours that could harm yourself, others, or could affect objects in the environment. This harm can be physical or psychological.
  • Aggression can be explained psychologically or biologically, and one of the main biological explanations is genes.
  • Genes may not directly affect aggression, but they alter certain neurotransmitters and hormones levels, which can cause chemical imbalances and other gene expressions, which in turn disturbs brain activity and may cause aggression
  • 3 ways of studying the genetic influence of genes on aggression are:
    1. Twin studies
    2. Adoption studies
    3. Gene-Environment Interaction studies
  • Twin studies, e.g. Coccaro et al. (1997), have shown that MZ twins have higher concordance rates (50% for physical assault and 28% for verbal assault) than DZ twins (19% for physical and 7% for verbal), which shows that genes do have an influence on aggression since MZ twins share a higher amount of their genes (100%) than DZ twins (50%).
  • Adoption studies compare adopted children's aggressive behaviour to that of their adoptive parents (who share the same environment) and to their biological parents (who share genes). Rhee and Waldman (2002) found that genes could account for 41% of the variance in aggressive behaviour.
  • Gene-Environment Interactions propose that since concordance rates aren't anywhere near 100%, then the environment must play a part in aggressive behaviour, too. Frazzetto et al. (2007) found that men with the MAOA-L gene variant were only more aggressive when they'd experienced early trauma (before 15 years old), while those who hadn't, didn't display aggressive behaviours (even if they possessed the MAOA-L variant).
  • This shows that genes create a disposition, and when environmental factors interact, the behaviour manifests.
  • There are strengths and weaknesses of twin and adoption studies regarding research support, validity, reliability, ethics, and generalisability.

Frequently Asked Questions about Genetics of Aggression

Gene's may not directly affect aggression, but they alter certain neurotransmitters and hormones levels, which can cause chemical imbalances and other gene expressions, which in turn disturbs brain activity and may cause aggression. Some examples of these are:

  • Serotonin
  • MAOA gene
  • XYY gene
  • Testosterone

There is evidence to suggest that genes do have an influence on aggression, but they aren't the only influence. This can be seen in the case of identical (MZ) and non identical (DZ) twins, as concordance rates would be close to 100% for Mz twins and 50% for Dz twins. 


But since they aren't (genetics account for only up to 50% of variance in aggression) it shows us that there are other influences on aggression. Some evidence that environmental factors interact with genes is provided by Frazzetto et al. (2007) who found that men with the MAOA-L gene variant were only more aggressive when they'd experienced early trauma (before 15 years-old), while those who hadn't, didn't display aggressive behaviours (even if they possessed the MAOA-L variant).  

Aggression has genetic origins in how the genes associated with aggressive behaviours are expressed. Namely, this can be seen in examples such as the MAOA gene. 

The two main parts of the brain involved in aggressive behaviour are the amygdala and the hypothalamus

Aggression can be learnt through operant conditioning, which involves positive and negative reinforcement, and reward and punishment. However, there is also a genetic component on aggression and research has found that genes can account for up to 50% of variance in aggressive behaviour.

Final Genetics of Aggression Quiz

Question

What is aggression?

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Answer

Aggression in psychology refers to behaviours that could harm yourself, others, or could affect objects in the environment. This harm can be physical or psychological.  

Show question

Question

What are the 2 types of studies used to investigate the genetic influence on aggression?

Show answer

Answer

Twin and Adoption studies

Show question

Question

What percentage of genes do Monozygotic (Mz) twins share?

Show answer

Answer

100%

Show question

Question

What percentage of genes do Dizygotic (Dz) twins share? 

Show answer

Answer

50%

Show question

Question

What were the findings of Coccaro et al. (1997) and what was the conclusion of their results?

Show answer

Answer

Coccaro et al. (1997) found that Mz twins have higher concordance rates (50% for physical assault and 28% for verbal assault) than Dz twins (19% for physical and 7% for verbal), which shows that genes do have an influence on aggression.

Show question

Question

How does researching with twins show whether there is a genetic influence on aggression or not?

Show answer

Answer

By comparing Mz and Dz twins we can see to what extent a behaviour is influenced by genes. This would be concluded by seeing if Mz twins had higher concordance rates of aggression than DZ twins. If they both had the same/similar concordance rates, that would mean that genes are not very influential in determining that behaviour since they'd have no difference in aggression despite the big difference in their proportion of shared genes. 

Show question

Question

What are concordance rates?

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Answer

Concordance Rate is the rate of likeliness that two people who share genes will develop the same disorder. 

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Question

What did Rhee and Waldman (2002) find in their meta analysis of adoption studies?

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Answer

They found that genetics could explain up to 41% of variance in aggression.

Show question

Question

How do we know that aggression isn't based only on genetics?

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Answer

While genetics have quite an influence, they aren't the only thing influencing aggression, otherwise we'd find concordance rates close to 100% for Mz twins and 50% for Dz twins. This means there's other factors that have influence.

Show question

Question

What did Frazzetto et al. (2007) find regarding gene-environment interactions?

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Answer

Frazzetto et al. (2007) found that men with the MAOA-L gene variant were only more aggressive when they'd experienced early trauma (before 15 years-old), while those who hadn't, didn't display aggressive behaviours (even if they possessed the MAOA-L variant).  

Show question

Question

What does it mean by having a 'genetic disposition to aggression'?

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Answer

Genes create a 'predisposition' (susceptibility) for aggressive behaviour, which then manifests when there is an interaction with the appropriate environmental factors.  

Show question

Question

What did McGue et al. (1992) find as supporting evidence?

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Answer

McGue et al. (1992) found that Mz twins had a higher correlation (+0.43) than Dz twins (+0.30) on aggression scales (measuring aggressive behaviour) 

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Question

Why is validity a strength for twin studies?

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Answer

Because Mz and Dz twins and the amount of genes they share (100% and 50% respectively) are a naturally occurring variable, in that researchers aren't manipulating anything, it means there's little to no researcher bias. 

Show question

Question

Why is validity a limitation for twin studies?

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Answer

Because Mz and Dz twins also share environments. This wouldn't be a problem if they shared their environments the same amount, but Mz twins share more of their environment than Dz twins since they are treated more similarly, even by their parents.

Show question

Question

Why is generalisability a weakness of adoption studies?

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Answer

We can't generalise the results from adopted children to other children because they have different experiences/environment (the reasons they were adopted e.g. unwanted by parents or dead parents) which can influence aggressive behaviour.

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