Genetic Factors

Question 1

Twin studies

Answer 1

Twin studies have suggested that genetic factors account for about 50% of the variance in aggressive behaviour.
Coccaro et al studied male monozygotic and dizygotic twins.
As MZ twins share 100% of their DNA and DZ twins share 50% we would expect to see greater similarities in aggressive behaviour between MZ twins if aggression is influenced by genetics. This is because MZ and DZ twins are raised in the same environment but MZ twins have a greater degree of genetic similarity.
Findings for physical violence – MZ - 50% concordance rate and DZ – 19% concordance rate.
Findings for verbal aggression – MZ – 28% concordance rate & DZ - 7% concordance rate.

Question 2

Adoption studies

Answer 2

Similarities in aggressive behaviour between an adopted child and their biological parents suggests that there is a genetic influence.
Similarities between an adopted child and their adoptive parent suggests that there is an environmental influence.
Rhee and Waldman’s meta-analysis of adoption studies found that genetic influence accounted for 41% of the variance in aggression – similar findings to twin studies.

Question 3

MAOA gene

Answer 3

The MAOA gene controls the production of an enzyme called monoamine oxidase A (MAO-A). The role of MAO-A is to break down serotonin.
A defective low-activity variant of the gene (MAOA-L or the ‘warrior gene’) produces low levels of the enzyme. This alters serotonin levels in the brain and leads to increased aggression.
Brunner et al found that 28 men from a Dutch family were involved in impulsive aggressive behaviour – e.g. rape, attempted murder and physical assault. Found to have lower levels of the enzyme MAO-A and the MAOA-L gene variant. This suggests that the MAOA-L variant leads to higher aggression levels.

Question 4

+ P - RS for the role that the MAOA gene plays in aggression

Answer 4

E - Mertins et al conducted a study where men took part in a money-distributing game (half had the MAOA-L gene and the other half had the ‘normal’ MAOA gene).
E - He found that those with the high activity variant were more cooperative and made fewer aggressive moves than the lower activity variant.
L - This suggests that there is a relationship between MAOA gene activity and aggression.

Question 5

• P - Twin studies may lack validity

Answer 5

E - In every pair of twins, both individuals share the environment - raised together.
E - However, DZ twins may not share their environment to the same extent that MZ twins share theirs. MZ twins are treated more similarly, encounter more similar environments (i.e. more likely to do things together) experience more ‘identity confusion’ (i.e. being treated as ‘the twins’ rather than as two distinct individuals) and are always same-sex whereas DZ twins can be a male/ female combination and are treated more like any other sibling pair.
L - Therefore, this means that concordance rates are inflated through environmental influence and so genetic influences or aggression may not be as great as twin studies suggest.

Question 6

• P - Precise mechanism of the serotonin-aggression link is unclear

Answer 6

E - The MAOA gene causes low activity of the MAOA enzyme which should lead to higher serotonin - because the low activity enzyme does not activate serotonin, leaving more serotonin fro synaptic transmission.
E - This goes against the neural explanation, which suggests that low levels of serotonin result in aggression.
L - Therefore, this shows that the relationship between the MAOA gene, serotonin and aggression is not yet fully understood.