Genetic Factors in Aggression

Question 1

Can your genes make you violent?

Answer 1

• All men in a violent family had the same genetic chain of MAOA, which was making MAOA useless.
• Warrior gene is associated with the breakdown of serotonin, and when not working, it may cause more impulsive behavior.
• New Zealand study found having a variation that lowers activity of MAOA was associated with a history of violence, but, mainly in the bodies of people with a history of childhood violence.
• Finland study, 2/3 of 800 prisoners had committed violent crimes. All had more MAOA.

Question 2

Twin Studies

Answer 2

• Several twin studies have suggested heritability accounts for about 50% of the variance in aggression.
• Caccaro et al. (1997) studied men who were either MZ or DZ twins.
• It was expected to find more concordance in MZ twins for aggression due to them sharing 100% of genes if genetic factors were a main cause of aggression.
• For direct physical assault, concordance rated was 50% for MZ twins and 19% for DZ twins.
• For verbal aggression, 28% MZ and 7% DZ.

Question 3

Adoption studies

Answer 3

• Adopted children and their genetic parents having similarities indicates genetic influence, whereas when with adopted parents, it indicates environmental influence.
• Rhee and Waldman (2002), meta-analysis of adoption studies of direct aggression and antisocial behavior.
• Found that genetic influences accounted for 41% of variation in aggression, in line with the twin studies.

Question 4

The MAOA Gene

Answer 4

• MAOA gene controls production of the monoamine oxidase enzyme.
• Monoamine Oxidase (MAO-A) regulates serotonin.
• Genes have variants, and the MAOA-L, low activity MAOA gene, is linked with greater aggressive behavior due to low activity of the enzyme affecting serotonin.
• Lea and Chambers (2007) showed that MAOA-L was possessed by 56% of the Maori men in New Zealand, famous for being ferocious warriors.
• Brunner et al (1993) studied 28 men from a Dutch family who had been involved in several aggressive violent crimes. These men had low levels of the enzyme MAO-A as well as the MAOA-L gene variant.

Question 5

Gene-Environment Interaction

Answer 5

• MAOA-L gene activity only related to adult aggression when combined with early traumatic life experiences.
• Frazetto et al. (2007) found an association between higher levels of antisocial aggression and the MAOA-L gene variant in men, but only in those who had experienced significant trauma in the first 15 years of life.
• Strong evidence of a gene-environment interaction.

Question 6

Research Support eval

Answer 6

• Mertins et al. (2011) made men with low and high activity variant of the gene take part in money distributing game.
• Found that high activity men we’re more cooperative and made less aggressive moves than low activity men.
• Supports relationship between MAOA-L and aggression.
• Also however showed environmental influence.
• MAOA-L men became more cooperative when told others were being cooperative, showing the influence of a social norm on aggression.

Question 7

Complex link

Answer 7

• Research says low serotonin is associated with aggression.
• However, MAOA-L leads to high serotonin, which should in turn lead to less aggression.
• May be more accurate to say that serotonin levels are disrupted not lower or higher.
• Link is not yet fully understood.

Question 8

Twin studies may lack validity

Answer 8

• DZ twins don’t share the same extent of the environment as MZ twins, challenging the equal environments assumption.
• MZ twins treated very similarly by parents, DZ treated less similarly.
• Suggests concordance rates are inflated, weakens the credibility of the genetic explanation.