Genetic factors in aggression

Biological psychologists ask:

“Do genetically related individuals have more similar tendencies of aggression than

non-related individuals?”

Twin studies 

Identical twins (monozygotic (MZ)) share all genes. Non-identical twins (dizygotic (DZ)) share 50% of genes.

Researchers use twin studies to see the level of similarity for a certain trait (e.g. aggression) of MZ twins and DZ twins. They can then compare these similarities.

MZ are more alike in terms of their aggression (this is due to genes, not environment as both types of twin share the same environment, but the MZ twins are more genetically similar).

Cocaro et al (1997) studies aggressive behaviour using adult twin pairs - almost 50% of aggression directed towards others could be attributed to genetic factors.

Adult twins aren't the same as children due to the development of the frontal lobe 

Adoption studies 

children and in their biological parents = genetic effect!

Positive correlation found between adoptee’s aggressive behaviour and adopted family = environment effect!

Hutchings and Mednick (1975): 14,000 adoptions in Denmark. Significant amount of adopted boys with criminal convictions had biological parents (especially fathers) who had been convicted of violent crimes

- evidence for the genetic effect!

Research into genetic factors in agression 

• Miles and Carey (1997)_ meta-analysis of 24 twin studies and adoption studies showing the genetic effect on aggression.

Findings:

• Strong genetic influence.

• Age also important (Younger = genes and environment both influence aggression.

Older = environment less influential, genetics

more influential).

• Rhee and Waldman (2002): 51 twin and adoption studies results were combined.

Finding:

• Aggressive, anti-social behaviour is mainly genetic.

The role of MAOA (AKA little hover)   

Monoamine Oxidase (MAOA): an enzyme which sweeps up any serotonin left in the synaptic gap after neurotransmission.

Without it, there would be a build up of serotonin in the synapse.

Brunner found a mutation in the X chromosome of the gene responsible for producing MOMA for male exhibiting aggressive behaviour. In rats who also had this mutation, their urine showed higher than normal levels of serotonin. The fact that the mutation for MAOA is on the X chromosome can also help explain why aggression is seen more in males than females. Males only have one X chromosome whereas women have two X chromosomes, so if the female's second X chromosome is ‘normal’ they won't experience the defect on the first.

Brunner (1993) studied a family in the Netherlands. Found males were mostly violent, aggressive and many had committed serious crimes (arson, rape). They had abnormally low MAOA due to a defect in the gene that produces it. Lack of further research. Only men, no women. Not representative as it could have just been found it ONE family 

MAMO-H and MAMO-L 

• MAOA-H (variant of MAOA) associated with high levels of MAOA.

• MAOA-L (variant of MAOA) associated with low levels of MAOA.

• Caspi (2002): study of 500 boys found a link between MAOA and aggressive behaviour. 

• boys with MAOA-L variant who had been mistreated when they were younger = much more likely to grow up to show anti-social behaviour.

• Boys with MAOA-H variant who were also mistreated and boys with MAOA-L who weren’t mistreated = no signs of anti-social behaviour. Contradicts the bio argujent as it suggests nurture does have an effect on aggression 

The warrior gene n 

MAOA-L - more common in those with a history of warfare (approximately 2/3 of people in this population), whereas only about 1/3 of people in Western populations have MAOA-L. It is therefore called the warrior gene 

McDermott (2009): pps with MAOA-L had higher levels of aggression MAOA-H pps (this was shown by their willingness (or lack of) to force a fictional pp to eat unpleasantly hot and spicy sauce).

Evaluations 

Problems of sampling 

• Samples are often individuals who have committed violent crime 

• Convictions for violent crime are relatively low compared to violent crimes that don't result in a conviction → represent a small minority of those involved in aggressive crimes/ behaviour 

• Criminals who are ‘violent may not be the most aggressive  → just labeled by the courts eg murder will be labelled as violent but may have avoided crime the rest of their life 

• Therefore this might explain why many studies have found little to no evidence 

More than one gene/factor 

• Behaviour is usually due to more than just one factor eg environmental 

• Genetic and environment interact with each other 

• Therefore it is hard to identify the exact contribution of genetic factors 

There are inconsistencies in the methods used to study genetic factors in aggression.

e.g. Miles and Carey’s meta-analysis showed that the methods used potentially influenced the outcomes.

• Use of parental-reports or self-reports

= aggression mainly due to genetic factors.

• Use of observations = aggression mainly due to environmental factors.

THEREFORE, inconsistent findings make it hard to assess relative contributions of genetic and environmental factors in aggression.

Influence of the MAOA gene can be supported with evidence

tiihonen et al. (2015): Finnish prisoner pps. MAOA-L combined with another gene (CDH13) was linked to extremely violent behaviour.

No solid evidence for either of the genes in non-violent offenders, so the combination seems to be specific for only violent offending.

HOWEVER, these genes do not predetermine violent behaviour, even if they make it more difficult for some to control violent urges.

Advantage of research into the MAOA gene is it can help to explain the gender inequality of violence.

Niehoff (2014): the genetic vulnerability to MAOA gene varies for males and females - males are more vulnerable to it than females as they only have 1 X chromosome, whereas females have 2.

HOWEVER, this could help to explain why males usually show more aggressive behaviour than females.