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In this research study, Swedish born citizens are compared to economic migrants and refugees, known to be prevalently males in their early twenties. 
Mean age of the Swedish born group is thus higher than the mean age of the other groups, and gender prevalence also differs substantially.
Since schizophrenia afflicts much more young males, the groups compared are not properly balanced.
Furthermore, the largest of the two foreign born groups is composed of poor economic migrants, not qualified for refugee status, who left their Country of origin at a young age, because they were not competitive, productive, adaptive, educated or qualified enough to fit into the local workforce.
Their psychosis could very well be a pre existing condition, expressing symptoms and behaviours that discouraged local employers from hiring them, rather than a consequence of their immigration abroad.
Such marginalized, underprivileged, deprived immigrants are intrinsically statistically expected to manifest more mental health issues than the general population of Sweden.