If researchers find a greater similarlity in levels of aggression between adopted children and biological parents it would suggest an important genetic influence
If children are more similar to their adoptive parents than their biological prents it would suggest environment has more of an influence
Hutchinsgs and Mednick reviewed 14000 adoptions. They found significant positive correlations betwen the number of convictions for violent criminals among biological parents.
Suggesting that biological influence is greater than environmental.
Rhee and Waldman carried out a meta-analysis of 51 twins and adoption studies using 87000 individuals.
They calculated that the genetic influence over the studies was approximately 40% and 60% was environment contribution.
There were little gender differences found.
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