We began our model fitting with a basic 1–1–1 model with specifics (where the first, second and third numbers reflect the number of genetic, shared environmental and unique environmental common factors). We then tested for quantitative and qualitative sex effects. Then, we sought to simplify the resulting model by deleting one by one all the common factors, and then the criterion-specific genetic and common environmental effects. (We did not attempt to eliminate the criteria-specific unique environmental effects as these include errors of measurement and it would be unrealistic to set these parameters to zero). Next, we made the model progressively more complex, whereas systematically searching at each step to find a ‘best-fitting’ model.
