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 practice, unreliability places an upper limit on any estimates of genetic effects. Thus the loading on the genetic common factor for disorder A may be higher than disorder B because we can measure disorder A much more reliably. Our second measure, the proportion of genetic variance shared with the common factor, ‘corrects’ for unreliability as its effects would be similar in the numerator and denominator of the ratio, hence canceling out.
