This study confirms the hypothesis that the majority of patients with psychotic disorders had been help-seeking for other mental disorders in the secondary mental health care prior to the onset of psychosis.

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The incidence of schizophrenia was three times higher in a longitudinal register study than in a high-quality first-contact study conducted in the same population. Risk estimates based only on first-contact studies may have been affected by selection bias.

PurposeTo estimate the effect of selective sampling on first contact (FC) studies of the relation between migration and schizophrenia.MethodsWe compared the FC method directly with a more inclusive longitudinal psychiatric register (LPR) method, by letting both methods estimate age and sex adjusted incidence rate ratios (IRR) in the population of The Hague aged 20–54 years, for the three largest migrant groups (first and second generation Caribbean, Turkish, and Moroccan) relative to the native Dutch population.ResultsBoth methods found that the adjusted IRR was higher for migrants than for native Dutch [all migrants IRR = 1.70 (95% Cl 1.30–2.21) for the LPR method and 1.91 (95% Cl 1.15–3.25) for the FC]. The IRR for Moroccans was significantly lower in the LPR [IRR 2.69 (95% 2.10–3.41)] than in the FC study [4.81 (3.41–6.68)]. The FC method was relatively more inclusive for migrants presenting at earlier ages or with shorter durations of prior treatment (DPT) than the native Dutch. This resulted in differential sampling and artificially higher IRRs for Moroccan and, to a lesser extent, Turkish migrants.ConclusionWe confirm that the incidence of schizophrenia is raised twofold for migrants compared to nonmigrants. Using the LPR method, however, IRR estimates were less pronounced for most migrant groups than in a high quality FC study conducted in the same population. The FC method may overestimate the risk of schizophrenia for migrant groups who seek first mental health at a relatively younger age, or who present directly with schizophrenia.Electronic supplementary materialThe online version of this article (doi:10.1007/s00127-016-1310-8) contains supplementary material, which is available to authorized users.

The mortality of enslaved Africans in the Atlantic crossing has long preoccupied historians but the relationship between slave traders' purchasing strategies and slave mortality rates in transit has escaped close investigation. We address these issues by using records of 39 eighteenth-century voyages of the Dutch Middelburgsche Commercie Compagnie. These allow shipboard mortality rates of enslaved Africans to be estimated. They also reveal previously un-noticed age- and gender-based variations in slave purchase and mortality patterns, which in turn shed light on the relative importance of African and shipboard conditions in determining slave survival rates in the middle passage.