Increased occurrence of schizophrenia and other psychiatric illnesses among twins

Background. We wished to establish whether the occurrence of schizophrenia among twins is the same as among the general population. Method. Through record linkage between the New Danish Twin Register and the Danish Psychiatric Case Register, the twins who have ever received a diagnosis of schizophrenia were identified. Through indirect standardisation for age, sex and dates of admission, the rate of first admission with diagnosis schizophrenia among twins is compared to the same rate in the general population. Results. A 28admission rate of any psychiatric disorders. Conclusion. This increased risk does not seem to be explained by known methodological problems, but does differ from earlier studies.