The Threshold Model
The threshold model was first applied to behavior by Irving Gottesman and James Shields in their classic twin study of schizophrenia. They postulated that a number of different genes along with a number of environmental variables acted as risk and protective factors for the development of schizophrenia. When enough risk factors accumulate and greatly outweigh the protective factors, then schizophrenia may develop. Because many factors contribute to schizophrenia, the liability or predisposition or vulnerability towards schizophrenia is really a continuous, quantitative dimension. Once, however, that liability (or predisposition or vulnerability) passes a certain point (the threshold), then a discrete phenotype emerges (affected with schizophrenia versus unaffected with schizophrenia).
This series of exercises will introduce you to the threshold model. Do them as they are assigned in the course, so do not try to do them all at once--some require information that will be covered later.
Threshold I: Introduction to the Threshold Model
Threshold II: Heritability and the Threshold Model
Threshold III: Effects of a Changing Environment
Threshold IV: The Correlation in Liability
Threshold V: Estimating the Correlation in Liability