AO1
There have been studies looking at aggression in MZ twins (identical twins, sharing 100% genes) and DZ twins (non-identical twins, sharing 50% genes). If MZ twins have similar levels of aggression then it should be due to genes and not the environment. Rutter carried out a meta-analysis on twins for criminality, he found a concordance rate of 13% - 22% for DZ twins and 26% - 51% for MZ twins.
AO2
This suggests that aggression is heavily influenced by genetics, as it was higher in MZ twins, who share 100% genes compared to DZ twins who share 50% of their genes.
However, as it is not 100% it cannot be purely due to genetics.
Furthermore, MZ twins could be treated more similarly, as they look a like.
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