Fertility of Patients With Schizophrenia, Their Siblings, and the General Population: A Cohort Study From 1950 to 1959 in Finland

OBJECTIVE: Genetic factors are the most important risk factors for schizophrenia. However, despite the fact that patients with schizophrenia have significantly fewer offspring than the general population, schizophrenia persists. The authors investigated whether the siblings of patients with schizophrenia produce more offspring, thereby compensating for the low fertility of the affected individuals. METHOD: From all 870,093 individuals born in Finland from 1950 to 1959, the authors determined how many had schizophrenia or were siblings of schizophrenia patients and how many offspring they had. The population data were obtained from the Population Register Center of Finland, and the National Hospital Discharge Register was used to identify all persons who had been hospitalized because of schizophrenia. Appropriate regression models were used to model age at the birth of the first child, number of children, and proportion of males among offspring. RESULTS: Of the total population, 1.3% were patients with schizophrenia, and 2.8% were their siblings. The mean number of offspring among female siblings was slightly but significantly higher than among women in the general population (1.89 versus 1.83), while the opposite was true for the male siblings (1.57 versus 1.65 among men in the general population). The mean number of offspring among patients with schizophrenia was 0.83 for women and 0.44 for men. CONCLUSIONS: Lower than average fertility among patients with schizophrenia is not compensated for by higher fertility among their siblings. Thus, the persistence of schizophrenia in the general population is not explained by this simple evolutionary mechanism.