Sibship characteristics during upbringing and schizophrenia risk.

The potential association between sibship characteristics and the risk of schizophrenia has been investigated previously. However, methods have differed and results have been conflicting. The authors explored the association between birth order, sibship size, and birth interval to siblings and schizophrenia while accounting for potential confounders. Using Danish Civil Registration System data, the authors established a cohort of 763,000 people. Schizophrenia was identified by linkage with the Danish Psychiatric Central Register. Overall, 2,536 people developed schizophrenia from 1986 to 2001. Children with no siblings and children with three or more siblings had significantly increased risks (relative risk = 1.22 and 1.27, respectively). This association was explained by change of residence and urbanization during upbringing. Children with half siblings had a significantly increased risk (relative risk = 1.20). The authors found a significantly increased risk associated with siblings 7-8 and 11-14 years younger (relative risk = 1.30 and 1.22, respectively) combined with a significantly decreased risk associated with siblings 2-10 and 12 or more years older (relative risk = 0.92 and 0.82, respectively). There was no consistent pattern between the birth interval to siblings and schizophrenia risk. Results of the association between sibship characteristics and schizophrenia from a single country may not be comparable with results from other countries, and sibship characteristics are minor determinants of schizophrenia risk.