The assumptions and properties of the Multifactorial Model of Disease Transmission are described, using the principles of quantitative genetics. No assumptions are made about the relative importance of genetic and environmental factors in the transmission of a disorder. The usefulness of the Model is discussed as a device for quantifying family data, discriminating between different modes of transmission and studying variations in disorders according to such clinical variables as age, sex, and severity of illness.
When only one form of a disorder can be distinguished, the degree to which the disorder is transmissible can be measured, and the relative importance of familial factors in the aetiology can be assessed. The assumptions which underlie the Model cannot be tested.
When two or more forms of a disorder can be distinguished, family and population data can be used to test the underlying assumptions of the Model. A good fit of the Model to the observed data indicates that the two subforms of the disorder represent different degrees of the same normally distributed process. This implies that the relative importance of transmissible and non-transmissible factors is the same in determining each subform of the disorder. The testability of the Multifactorial Model with multiple thresholds offers an opportunity for relating and understanding heterogeneous subforms of a disorder, for specifying the relationship between spectrum disorders, and for classifying psychiatric disorders.