Genetics of Aggression

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.

Aggressive Behaviour, freepik.com

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.

Mz 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.

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).

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.

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.

Genetic Predisposition to Aggression

Considering the above studies, they 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 on aggression.

Research Support

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

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. However, Twin Studies may lack validity because while twins do share genes, they also share environments too since they grow up together (in most cases).

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

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

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.

Ethics

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. 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.

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

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.