To the Editor.—
When a psychiatric disorder has been shown to aggregate (or "run") in families, the psychiatric geneticist usually wants to determine the degree to which that aggregation results from familial/environmental vs genetic factors. Twin studies may be used to address this question. The interpretation of such studies has usually focused on the comparison of concordance rates (probandwise concordance is defined as the risk of illness in the cotwin of a proband-twin) in monozygotic (MZ, or identical) and dizygotic (DZ, or fraternal) twins. While a significantly higher concordance rate in MZ vs DZ twins is strong evidence for the existence of some degree of genetic involvement in the disorder under consideration, without a model for genetic and nongenetic familial transmission, it is not possible to make any more quantitative statements about the magnitude of such involvement.However, to circumvent the complexity of genetic models, several "model-free" "rules of thumb"