of schizophrenia and depression in the young, randomly selected iPSYCH-population-sample are 1% and 2.7%, respectively, based on register-based hospital diagnosis. Schizophrenia has relatively early onset, while depression has relatively late onset, hence the relatively low observed 2.7% frequency of depression in the random population-based sample of iPSYCH. Thus, the young iPSYCH sample is likely to capture relatively severe early-onset depression cases with a high risk for developing schizophrenia9. These factors explain the apparently high frequency of depression-cases with a schizophrenia co-diagnosis observed in the iPSYCH sample. As the iPSYCH cohort grows older, and more individuals develop depression, we will expect the frequency of schizophrenia co-diagnosis among depression cases to be reduced.
