MZ twins share 100% of their genes, while DZ twins share 50% of their genes.
If a researcher compares the similarity between sets of MZ and DZ twins of aggressive traits and finds that MZ twins are more alike in terms of this trait, then this 'extra' likeness should be due to genetic rather than environmental factors.
Aggressive behaviour is more correlated in identical twins (MZ) than fraternal twins (DZ).
Concordance rate is the chance of the behaviour/disorder occurring in both twins.
Twin Studies (AO2)
McGuffin and Gottesman (1985) - found a concordance rate of 87% for aggressive and antisocial behaviour in MZ twins compared with 72% in DZ twins is still high.
- This indicates that family environment (shared across siblings) has an important role in development of aggressive behaviour because the concordance rate in DZ twins is still high.
- This study also supports the fact that aggression has genetic links.
Twin Studies (AO2) part 2
The problem for researchers is disentangling the effects of nature and nurture.
Mason and Frick (1994)
- 12 twin studies.
- 3795 twin pairs.
- Concluded that aggression is 50% genes and 50% environment.
More violent behaviours are associated with genetic links - although there is a dual role in the onset of aggression.
It partially supports the theory/hypothesis.
Twin Studies (AO2) part 3
Cocarro et al (1997)
- 182 MZ twin pairs.
- 182 DZ twin pairs.
- 40% down to genes and 60% down to environment.
- Verbal aggression has stronger genetic links than physical aggression.
Twin Studies (AO2) part 4
Twin studies are based on assumptions which are not always met (AO2):
Twin studies assume that MZ and DZ twins raised in the same homes experience the same environments.
However, research shows that parents, teachers, peers and others may treat MZ twins more similarly than they do DZ twins. Therefore, twin studies are not the perfect 'genes vs. environment' experiment that they appear to be.
Some researchers believe that interaction between genes and environment is what can bring out aggressive traits, rather than genes alone.
Twin Studies (AO2) part 5
Difficulties in twin research
- It is very rare to find MZ twins reared apart and therefore separating the effects of nature and nurture is extremely difficult.
By studying children who have not been brought up by their biological parents researchers are able to separate effects of environment and genes.
If researchers find a greater similarity between adopted children and their biological parents than this suggests an important genetic influence is at work.
If researchers find a greater similarity between adopted children and their adoptive parents then this suggests important environment factors are at work.
Adoption Studies (AO2)
One of the largest ever adoption studies - Hutchings and Mednick (1973) - 14,000 adoptions in Denmark.
- Significant positive correlation between number of violent criminal convictions among biological fathers and their sons who had been adopted.
24 twin and adoption studies - Miles and Carey (1997)
- 50% genetic influence.
- Genetic influence becomes more prominent in adolescence.
Adoption Studies (AO2) part 2
Rhee and Watson (2002)
51 twin and adoption studies - 87,000 individuals.
- Genetic component of anti-social behaviour is 40% and environment is 60%.
- No gender differences.
- Self-reports and reports from others found different results (39% vs 53%) - which suggests that the METHOD of assessing aggression affects the results significantly.