Increased occurrence of schizophrenia and other psychiatric illnesses among twins.
U Kläning, P B Mortensen, K O Kyvik


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 28% increase in the rate of first admissions for schizophrenia in twins compared to the general population was found. This is statistically significant and does not seem to be caused by any known methodological problems. Twins also had an increased first-admission 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.