Genetic factors in aggression

Monozygotic twins (MZ)

Identical twins

Dizigotic Twins (DZ)

Non - identical

Why are MZ twins more alike in terms of aggression

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

studied aggressive behaviour using adult twin pairs - almost 50% of aggression directed towards others could be attributed to genetic factors.

CP - Adult twins arn’t the same as children due to the development of the frontal lobe

What type of correlation was found between adoptee’s aggressive behaviour and adopted family … and what does this suggest

Positive correlation which suggests there is also an environmental effect

Hutchings and Mednick

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

Supports the genetic effect

Miles and Carey

Conducted a meta analysis of 24 twin studies and adoption studies showing the genetic effect on aggression

They found there was a strong genetic influence and that age was an important factor

eg younger = genes and environment both influenced aggression while older = environment was less influencial

Rhee and Waldman

51 twin and adoption studies were combined and found that aggressive, anti-social behaviour is mainly genetic

What is MAOA

An enzyme which sweeps up any serotonin left in the synaptic gap after neurotransmition. Without it there would be a build up of serotonin in the synapse

Brunner

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. —> shows there was a genetic influence

C.P - Lack of further research. Only men, no women. Not representative as it could have just been found it ONE family 

Brunner and the X chromosome

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.

What are the 2 variants of MAOA

MAOA-H (high levels of MAOA)

MAOA-L

• 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.

  C.P- Contradicts the bio argument as it suggests nurture does have an effect on aggression 

What gene in the warrior gene

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).

AO3 - Problems with 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 labelled 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 of heritability for violence 

Difficulties of determining the role of genetic factors - AO3

• 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 

Inconsistencies in the methods used to study genetic factors in aggression. - AO3

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 - AO3

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. - AO3

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.