who share on average 50% of their genes. For genetically influenced behaviour we would expect greater MZ than DZ similarity. In the basic (ACE) model, variation is assumed to arise from three sources: (1) additive genetic effects (a2); (2) common environmental effects (c2); and (3) unique environmental effects (e2). Common environmental effects serve to make twins more similar to one another, while unique environmental effects tend to make the individuals in a twin pair less similar [37].
