Can the new epidemiology of schizophrenia help elucidate its causation?

The supposed universality of the incidence and prevalence of schizophrenia has been seriously challenged. It is now widely accepted that the life-time prevalence and incidence of this disorder vary considerably in time and place. As a result, there has been renewed interest in environmental causation of schizophrenia. There are few extant formulations that have successfully integrated the available new evidence into a coherent theory for its causation. The outgroup intolerance hypothesis is an attempt to integrate this evidence. It proposes that schizophrenia is the result of a mismatch between the social brain as shaped by evolution and the new social conditions of the post-neolithic. The hypothesis can provide an explanation for (i) the higher risk to migrants, (ii) the ethnic density phenomenon, (iii) the increased risk to individuals who have grown up in cities and (iv) the putative low risk in hunter-gatherer societies. Evidence is presented from a range of disciplines and sources including epidemiology, psychopathology, social psychology and clinical trials in support of this hypothesis. A range of testable predictions follow from the hypothesis.