With a sample of twins reared together that includes opposite sex DZ pairs, it is possible to test for qualitative and quantitative gender differences in the aetiology of behaviour [37]. A sex limitation model that includes qualitative gender differences is referred to as a ‘general effects model’ and can be specified to assume a (partially) different genetic or common environmental aetiology for males and females. A sex limitation model that includes quantitative gender differences but no qualitative gender differences (i.e. has the same genetic and environmental aetiology but differences in magnitude for males and females) is referred to as the ‘common effects model’. A model with no gender differences (i.e. the genetic and environmental estimates for males and females are equated) is referred to as a ‘sex homogeneity’, or ‘no sex effects model’ [37]. By comparing the fit of minus twice the log-likelihood and the change in degrees of freedom between these models it is possible to test for significant gender differences in aetiology. However, the two alternate forms of the general effects sex limitation model (i.e. qualitative differences in
