Twin studies have provided evidence that aggression is inherited. Read the extract on the handout on 'Twin Studies'.
Some studies have found a higher correlation in aggressive behaviour between monozygotic twins than dizygotic twins. McGuffin and Gottesmann (1985) found a concordance rate of 87% for aggressive and antisocial behaviour for MZ twins, compared with 72% for DZ twins.
Is this convincing evidence for the role of genetic factors in aggressive behaviour? What does the finding that the concordance rate (if one twin is aggressive the other twin is aggressive as well) for DZ twins is 72% suggest? What basic problem does this cause for twin studies investigating the contribution of genes to aggressive behaviour?
A meta-analysis by Mason & Frick (1994) of 12 twin studies involving 3795 twin pairs estimated 50% of the difference between anti-social and non-antisocial behaviour could be due to genetic factors. The effect was even larger for violent behaviour. Identify one problem with this estimate for the inheritance of aggressive behaviour.
Coccaro et al (1997) conducted a study on aggressive behaviour that attempted to estimate the influence of genetic and environmental factors on aggression in male participants . This study analysed data on aggression from 182 MZ twin pairs & 118 DZ twin pairs. They estimated that genes accounted for 40% of the individual differences in aggression. Environmental influences accounted for 50% of individual differences in aggression. Identify one issue this study may raise.
A meta-analysis of twin studies by Rhee and Waldman (2002) supported the estimation by Coccaro et al (1997) that aggression has a genetic component of 40%. Read the extract on the handout 'Meta-analysis of twin and adoption studies' and answer the questions. Read the extract 'Evaluation of twin and adoption studies'. What is the main methodological problem identified with most twin studies?
Adoption Studies One major problem with twin studies is that each twin shares the same physical, family, and social environment. This makes it difficult to estimate accurately the contribution of environmental factors and genetic factors on aggression. This is why research that looks for correlations in behaviour between twins separated in early life are highly valued.
Adoption studies also look for correlations in aggressive behaviour between children raised by adults who are not their biological parents and their biological parents. If adopted children are similar in behaviour to their adoptive parents than their biological parents this is strong evidence that environmental factors are more important than genetic factors: nurture is more important than nature in the development of aggressive behaviour.
Hutchings & Mendick (1973) investigated 14,000 adoptions in Denmark and found a significant positive correlation between the number of convictions for criminal violence among the biological parents ( fathers ) and criminal convictions among their adopted children ( sons ).
Miles & Carey (1997) conducted a meta-analysis of 24 twin & adoption studies studies and found that genetic factors accounted for 50% variance in aggression. They also found that genetic factors had a stronger influence on aggression as the twins and adopted children got older. This is strong evidence that over time genes win out over environmental factors over time. Read the extract 'Problems with the interpretation of adoption studies' and identify the main problem with the validity of adoption studies as a measure of the influence of genes on aggression.