Familial links between schizophrenia and other disorders: application of the multifactorial polygenic model

A substantial number of patients do not meet diagnostic criteria for schizophrenia yet exhibit psychotic phenomena characteristic of that disorder (e.g., schizophreniform disorder, schizoaffective disorder, paranoid disorder, atypical psychosis). The relationship between schizophrenia and these disorders is poorly understood. This paper describes how the two-threshold multifactorial model of familial transmission can be applied to test the hypothesis that these disorders share a multifactorial etiology with schizophrenia. The procedure is illustrated using data from a blind family study of the major psychoses. Results support the hypothesis that the disorders under examination share a common multifactorial familial etiology. Methodological issues are discussed along with suggestions for future research in this area.