Background: Twins are exposed to intrauterine environments that differ significantly from those of singletons. These diverse environments might alter the risk for schizophrenia in twins and make it difficult to generalize from findings in twins when studying the risk of schizophrenia in the general population. Previous studies report contradictory findings on the risk for schizophrenia in twins. Methods: We studied the incidence of schizophrenia spectrum disorders, ascertained from Israel's National Psychiatric Registry, in a cohort of 2124 twins and 87,955 singletons. These offspring were followed from their birth in 1964-76 in the Jerusalem Perinatal study. Cox proportional hazards methods were used to compare outcomes over 28-41 years, adjusting for ages of parents. Results: Twins showed a relative risk [RR] of .84 relative to singletons, with a 95% confidence interval [CI] of (.51-1.4). RRs and CIs for males and females were .68 [.34-1.4] and 1.1 [.55-2.2] respectively. Twins in male-male, female-female or opposite-sex sets showed no significant variation in RRs; furthermore, first- or second-born twins did not differ significantly from each other. Siblings of twins had the same risk of schizophrenia as siblings of singletons. Conclusion: Twins have the same risk for schizophrenia as the general population.
- Maternal age
- Paternal age