Genetic Factors In Aggression

What have several twin studies suggested?

That heritability (genetic factors) account for about 50% of the variance in aggressive behaviour.

What did Coccaro et al. study?

Men who were either monozygotic or dizygotic twins. Because MZ twins share 100% of their genes but DZ twins share only 50% (on average), we would expect to find greater similarities between MZ twins if aggression is mostly influenced by genetic factors, as both MZ and DZ twins are raised in the same environment.

What did Coccaro et al. find?

They found concordance rates of 50% for MZ twins and 19% for DZs. The corresponding figures for verbal aggression were 28% (MZs) and 7% (DZ).

What do similarities between an adopted child an their biological parents suggest?

That genetic influences are operating.

What do similarities between an adoptive child and their adoptive parents suggest?

That environmental influences are operating.

What did Rhee and Waldman carry out?

A meta-analysis of adoption studies of direct and antisocial behaviour, a prominent feature of which is aggressive behaviour.

What did Rhee and Waldman find?

That genetic influences accounted for 41% of the variance in aggression, more or less in line with findings from twin studies.

What is the MAOA gene?

The gene responsible for the activity of the enzyme MAO-A. The low activity variant is closely associated with aggressive behaviour.

What does MAO-A regulate?

The neurotransmitter serotonin, which is thought to play an important role in impulsive aggression.

What does the low activity variant (MAOA-L) result in?

Low activity of the MAO-A enzyme, and has been linked to aggressive behaviour.

What did research by Lea and Chambers on the MAOA ‘warrior‘ gene find?

That the low activity variant was possessed by 56% of New Zealand Maori men (compared with 34% of caucasians). Historically, the Maori people have had a reputation for being ferocious warriors.

What was the link between MAOA-L and aggression further cemented by?

Brunner et al.

What did Brunner et al. study and find?

28 men from a large Dutch family who were repeatedly involved in impulsively aggressive violent criminal behaviours such as rape, attempted murder and physical assault. These men had abnormally low levels of MAO-A as well as the MAOA-L gene variant.

While genes are crucial influences on aggressive behaviour, do they function in isolation?

No, it appears to be the case that MAOA-L gene activity is only related to adult aggression when combined with early traumatic life experiences.

What did Frazzeto et al. find an association between?

Higher levels of antisocial aggression and the MAOA-L gene variant in men, as expected. But this was only the case for those who had experienced significant trauma (such as sexual or physical abuse) during the first 15 years of life. Those who had not experienced such trauma did not have particularly high levels of aggression as adults, even if they possessed the MAOA-L variant.

What is the strength of the genetic explanation (the role of MAOA in aggression)?

Research support.

How is research support a strength of the genetic explanation?

There is support for the role of the MAOA gene in aggression. Research shows that the low-activity variant of the MAOA (MAOA-L) gene is associated with greater aggression.Mertins et al. found that the reverse is also true. Men with low-activity and high-activity variants of the MAOA gene took part in a money-distributing game. Men with the high-activity variant (MAOA-H) were more co-operative and made fewer aggressive moves than the low-activity participants.

What does the research support the relationship between?

MAOA gene activity and aggression.

What is the counterpoint to the research support?

The study by Mertins et al. also showed than non-genetic factors are crucial. They found that even participants with the low-activity MAOA variant (MAOA-L) behaved co-operatively rather than aggressively when they were made aware that others in the study were behaving co-operatively (i.e. giving away money). Knowledge of a social norm partly determined how aggressive or co-operative MAOA-L participants were.

What does the counterpoint to the research support mean?

That therefore, genes are influenced by environmental factors (knowing about others' behaviour) that are at least as important in aggression.

What are the limitations of the genetic explanation?

• Complex link.

• Problems with twin studies.

How is a complex link a limitation of the genetic explanation (MAOA-serotonin aggression link)?

The precise mechanism is unclear. Previously, research linked aggression with low levels of serotonin (e.g. Virkkunen et al.). But the MAOA-L gene causes low activity of the MAO-A enzyme which in turn should lead to higher serotonin - because the low-activity enzyme does not deactivate serotonin (its normal function), leaving more serotonin for synaptic transmission. In people with the MAOA-L variant, it may be more accurate to say that their serotonin levels are disrupted rather than they are lower or higher than normal.

What does the complex link show?

That the relationship between the MAOA gene, serotonin and aggression is not yet fully understood.

How is problems with twin studies a limitation of the genetic explanation?

Twin studies may lack validity: in every pair of twins, both individuals share the same environment as each other (because each pair is raised together). However, DZ twins may not share their environment to the same extent that MZ twins share theirs. We assume they do and this is called the equal environments’ assumption. But the assumption may be wrong because one aspect of the environment is the way twins are treated by others. MZ twins are treated very similarly, especially by parents (e g- praising them equally for being aggressive). DZs are treated in less similar ways.

What do the problems with twin studies mean?

That concordance rates are inflated and genetic influences on aggression may not be as great as twin studies suggest.