We examined the relationship between individual disorders and traits, and the genetic common factors in two ways. The most common-sense approach, emphasized in our discussion, is the coefficient on the path from the common genetic factor to the disorder. However, we also presented the proportion of disorder or trait heritability due to the common factor because this measure has at least one conceptual and one practical advantage. Conceptually, our commonsense approach ignores the impact of genetic effects specific to the disorder. Consider two disorders. The first has a 0.5 loading on the common genetic factor (so that 0.52=25% of the variance is due to genes shared with the common factor) but also has 25% of its variance due to disorder-specific genes. The second disorder has the same 0.5 loading on the common genetic factor and zero disorder-specific genes. Although the two disorders have the same magnitude of path from the common factor, in some senses, the genetic risk for the second disorder is a ‘purer’ index of the genetic common factor than is the genetic for the first disorder. In
