Twin - Studies

Berkowitz (1993) 

Studies from the 1930s found an average concordance rate of 75% for MZ twins and 24% for DZ. Support that genetics do influence levels of aggression. 


McGuffin and Gottesman (1985) 

Found a concordance rate of 87% for aggressive and antisocial behaviour for MZ twin pairs, compared with DZ twin pairs (72%). 
Findings also indicate that family environment, shared across siblings, exerts an important influence. 
Studies support the importance of genetic factors in aggression - but also environments that siblings share. 

Meta analysis by Mason and Frick (1994)

Procedure
- Analysed 12 twin studies involving 3795 twin pairs.

Findings 
- Approximately 50% of the difference between antisocial and non-antisocial behaviours could be attributed to genetic factors.

Coccaro et al (1997) 

In one of the twin studies, Coccaro assessed the degree of genetic and environmental influences on aggression in male participants. Data from 182 MZ twin pairs and 188 DZ twin pairs were analysed. 
Genes accounted for more than 40% of individual differences in aggression. 
Environment accounted for 50% of individual differences in physical aggression and 70% in verbal aggression.