Genetic Factors

Question 1

What have some twin studies suggested?

Answer 1

• Heritability (genetic factors) accounts for about 50% of variance in aggressive behaviour

Question 2

Coccaro et al (1997)

Answer 2

• Studied men who were either monozygotic (MZ) or dizygotic (DZ)
• Because MZ twins share 100% of their gene but DZ twins only share 50% on average, we would expect to find greater similarities in aggressive behaviour between MZ twins if aggression is mostly influenced by genetic factors – this is because both MZ and DZ twins are raised in the same environment, but MZ twins have a greater degree of genetic similarity
• The researchers found concordance rates of 50% for MZ twins and 19% for DZ twins (for aggressive behaviour defined as direct physical assault) and concordance rates of 20% of MZs and 7% for DZs (for verbal aggression)

Question 3

What do similarities in behaviour between child and parents suggest?

Answer 3

• Similarities in aggressive behaviour between an adopted child and their biological parents suggest that genetic influences are operating
• Similarities between an adopted child and their adoptive parents suggest that environmental influences are operating

Question 4

Rhee & Waldman (2002)

Answer 4

• Carried out a meta-analysis of adoption studies of direct aggression and anti-social behaviour, a prominent feature of which is aggressive behaviour
• They found that genetic influences accounted for 41% of the variance in aggression, similar to findings from twin studies

Question 5

MAOA Gene

Answer 5

• Controls production of an enzyme called MAO-A, which breaks neurotransmitters down (especially serotonin) into chemicals to be recycled
• The low-activity variant gene (MAOA-L) results in low activity of the MAO-A enzyme (MAO-A deficiency) – has been linked to high levels of aggressive behaviour
• The gene has been nicknamed ‘warrior gene’ because of research by Lea and Chambers (2007) which showed that the variant was possessed by 56% of New Zealand Maori men (compared with 34% of Caucasians)

Question 6

Brunner et al (1993)

Answer 6

• Studied 28 men from a large Dutch family repeatedly involved in impulsively aggressive violent behaviour like rape, attempted murder, and physical assault
• These men had abnormally low levels of the enzyme and the gene variant

Question 7

Stuart et al (2014)

Answer 7

• Studied 97 men who were part of a batterer treatment programme because they had been involved in inflicting intimate partner violence
• Men with the low activity gene were found to be the most violent perpetrators of intimate partner violence

Question 8

Gene-Environment (GxE) Interactions

Answer 8

• Genes are crucial influences but don’t function in isolation – MAOA-L gene activity is only related to adult aggression when combined with early traumatic life experiences

Question 9

Frazzetto et al (2007)

Answer 9

• Found an association between higher levels of antisocial aggression and the gene variant in men, but this was only the case in those who had experienced significant trauma during the first 15 years of life
• Those who had not experienced such childhood trauma did not have particularly high levels of aggression as adults, even if they possessed the variant gene
• This is strong evidence of a gene-environment interaction

Question 10

Evaluation - Role of MAOA gene in aggression

Answer 10

• Research shows that low-activity variant of the gene is associated with greater aggression, but Mertins et al (2011) also found the converse is true
• Men with low-activity and high-activity variants of the gene took part in a money-distributing game
• Men with the high-activity variant were more cooperative and made fewer aggressive moves than the low-activity participants
• Supports relationships between MAOA gene activity and aggression

Question 11

Evaluation - Study also showed that non-genetic factors are crucial

Answer 11

• They found that even participants with the low-activity variant behaved cooperatively rather than aggressively when they were made aware that others in the study were behaving cooperatively, meaning that knowledge of a social norm partly determined how aggressive/cooperative MAOA-L participants were
• Genes are influenced by environmental factors (knowing about others’ behaviour) that are at least as important in aggression

Question 12

Evaluation - Twin studies may lack validity

Answer 12

• In every pair of twins, both individuals share the same environment as each other, but DZ twins may not share the environment to the same extent that MZ twins share theirs
• The assumption that they do (equal-environments assumption) may be wrong because one aspect of the environment is the way twins are treated by others – MZ twins are treated very similarly, while DZs are treated in less similar ways, meaning that concordance rates are inflated and genetic influences on aggression may not be as great as twin studies suggest

Question 13

Caspi et al (2002) - Aim & Procedure

Answer 13

• Aim was to investigate how environmental stressors interact with our genes to produce aggressive behaviours
• Followed over 1000 children (52% of which were male) in Dunedin, New Zealand over around 25 years and took measures every few years
• Genetic makeup was examined, concentrating on MAOA gene; upbringing was also examined, noting maltreatment; at 26, the anti-social behaviour of participants was measured

Question 14

Caspi et al (2002) - Findings & Conclusion

Answer 14

• Those participants who had been abused as children were more likely to be antisocial; those who had the MAOA-L gene variant and were abused as children were more likely to be antisocial and aggressive adults, compared with those who had just been abused/had the gene
• 12% of participants had the gene + were abused in their youth; this accounted for 44% of the entire cohort’s violent convictions
• Overall, experiencing abuse and neglect as a child leads to anti-social behaviour as an adult; the ‘warrior gene’ alone does not cause anti-social behaviour but in combo with abuse as a child, it can increase