Aggression : Genetic Factors

Question 1

What are monozygotic twins?

Answer 1

Share 100% DNA (genetically identical – 1 single egg that spilt)

Question 2

What are dizygotic twins?

Answer 2

Share 50% DNA (normal siblings – 2 different eggs)

Question 3

MZ twin similarities and differences?

Answer 3

MZ twins share 100% DNA so any diffs are assumed to come from environment. Similarities are assumed to be bc of genetics.

Question 4

What is a concordance rate?

Answer 4

The degree of similarity is measured w a concordance rate (the presence of the same trait in both twins). Concordance rates are more commonly used in DZ twins.

Question 5

Coccaro et al (1997)

Answer 5

Studies specifically aggressive behaviour (physical assault) in adult twins. MZ twins had concordance rate of 50% and DZ 19% - component of aggression could be genetic.

Question 6

Verbal aggression study

Answer 6

Verbal aggression was 28% MZ and 7% DZ (most other twin studies have focussed on criminal behaviour generally and not specifically aggression. So, it is difficult to determine the validity of this study)

Question 7

Adoption studies

Answer 7

If a positive correlation is found between aggressive behaviour in adopted children and aggressive behaviour in their adopted parents then an environmental effect is implied. If the correlation is between the adopted child and their biological parents then a genetic effect is implied.

Question 8

Rhee and Waldman (2002)

Answer 8

meta-analysis of adoption studies of direct aggression and anti-social behaviour (prominent aggressive trait). Found genetic influences accounted for 41% of the variance in
aggression – the same finding as twin studies.

Question 9

Hutchins and Mednick (1975)

Answer 9

study on 14,000 adoptions in Denmark, found that a significant number of adopted boys with criminal convictions had biological parents (esp fathers) with criminal convictions.

Question 10

Brennan and Mednick (1993)

Answer 10

found that genetic influences were significant only in cases of property crime not in violent crime

Question 11

Whats the MAOA gene?

Answer 11

Controls production of the MAO-A enzyme

Question 12

Whats Monoamine Oxidase A (MAO-A)?

Answer 12

regulates the metabolism of monoamines (dopamine, noradrenaline and serotonin) as low levels of serotonin are associated with high levels of aggression

Question 13

Whats is MAOA-L?

Answer 13

Low variant of MAO-A enzyme, results in low activity (linked to high levels of aggression)

Question 14

What is the warrior gene?

Answer 14

MAOA - the low activity variant (MAOA-L) is seen more frequently in populations with a history of warfare – with 2/3rds of these populations having low activity versions. Eg: Lea and Chambers (2007) showed the MAOA-L variant was present in 56% of New Zealand Maori men (reputation for being aggressive warriors), vs 34% Caucasians.

Question 15

Brunner et al (1993)

Answer 15

studied a Dutch family where 28 of the males behaved very aggressively and violently (e.g. rape, attempted murder and assault). These men were found to have abnormally low levels of MAO-A enzyme and the MAOA-L gene variant (sometimes referred to a Brunner Syndrome).

Question 16

Stuart et al (2014)

Answer 16

Found that of 97 men convicted of intimate partner violence (IPV), those with the MAOA-L variant gene were the most violent perpetrators and this resulted in the worst injuries on their partners.

Question 17

Gene environment interaction

Answer 17

Genes have crucial role in agg, but don’t work alone. MAOA-L gene activity correlation w aggression combines with trauma (diathesis stress model).

Eg: Caspi et al (2002) studied 500 male children and found those with the MAOA-L gene are more likely to grow up and behave antisocially but only if they had also been mistreated as children. Children with MAOA-H who were also mistreated and those with MAOA-L who had not been misltreated did not display antisocial behaviour. This suggests it is an interaction between the gene and its environment that is important (diatheses- stress). Reliable as were also found by Frazzetto et al (2007).

Question 18

What are the strengths of genetic factors?

Answer 18

• Empiracle support for heritability in aggressive behaviour → Rhee & Waldman (2002) conducted a meta-analysis of twin and adoption studies, finding that genetic factors account for 41% aggression variance. Suggesting a substantial hereditary influence, which implies that aggression is partly genetic, not solely environmental. However, the 41% variance indicates the importance of other influences, like family dynamics, social experiences, and stress. These factors interact with genetic predispositions, meaning that while genetics contributes to aggression, environmental and situational factors are also crucial for a comprehensive understanding.

Question 19

What are the limitations of genetic factors?

Answer 19

• Difficulty measuring effects of genetics along and separating them from the environment → McDermott (2009) showed that Ps with MAOA-L, behaved aggressively in a money-allocation lab experiment but only when they were ‘provoked’. Otherwise they acted the same as MAOA-H, which shows that aggression is more likely explained by an interactionist approach (gene vulnerability).
• Multiple genes involved in aggression which makes it difficult to research → Stuart (2014) showed that intimate partner violence in men was not just associated with MAOA-L but also the serotonin transporter gene (5-HTT). It was a combination of both of these genes that was most linked to IPV. This shows that it is too reductionist to isolate one gene. The search for other genes is a very active body of research. Vassos (2014) could find no association between a singular gene and aggression in a meta-analyses; Tielbeek (2018) found that 40 genes were associated with aggression, in a study of 30,000 individuals.
• In many agg studies, most Ps are convicted of violent crime → For example, studies by Raine et al. (1997) and Coccaro et al. (2007) have identified limbic dysfunction in violent criminals. However, this raises two issues: first, convictions for violence are few compared to the number of violent acts resulting in a conviction. The sample is therefore biased to those convicted of violence. Secondly, offenders designated as violent based off conviction are not necessarily the most violent and persistent offenders (e.g. a one off murderer would be labelled violent even if no other violent history)