Objective: This study set out to determine, in a homogeneous sample with nationwide coverage in Finland, whether siblings treated for schizophrenia are more often of the same sex than expected by chance, and whether this is especially so when the disorder is transmitted by their fathers.
Method: Finnish social and health insurance files as well as hospital discharge registers were searched for probands with schizophrenia from a birth cohort spanning 30 years. Nuclear families were identified by cross-linkage with the national birth register, and the sex distribution observed in multiply affected sibships was compared with expected distributions by maximum likelihood analysis.
Results: In the subset of multiply affected sibships with one parent who had schizophrenia (84 fathers and 120 mothers), the observed sex distribution did not deviate from the expected pattern. However, a small and marginally significant excess of sex concordance emerged from the total sample of 1,942 sibships in which there were at least two affected members, irrespective of the parents' affection status.
Conclusions: The results indicate that no above-chance sex concordance in sibships multiply affected with paternally transmitted schizophrenia is present in the genetically homogeneous population of Finland. In view of a virtually unbiased and complete ascertainment procedure and sample sizes one to two orders of magnitude larger than those in previous studies, the authors attribute prior findings of such a concordance to sampling artifacts or chance fluctuations and finally conclude that except for regional genetic isolates, there is no epidemiologic evidence that a gene accounting for substantial susceptibility to schizophrenia in a greater proportion of cases resides in the pseudoautosomal region of the sex chromosomes.