Winter birth and biological family history in adopted schizophrenics

To investigate relationships between birth season and biological family his tory in schizophrenia, this study used a sample of schizophrenics that had the advantages of(a) particularly thorough diagnostic assessments of schizo phrenics' relatives, including information from direct interviews as well a s chart reviews, and (b) schizophrenic probands who were adopted at early a ge, mitigating the usual confounding of genetic and postnatal environmental influences of the family. Adopted schizophrenics with no biological family history of schizophrenia-spectrum disorders were significantly more likely to be born in winter months than were either (a) their own biological rela tives, including their sibs and half-sibs, (b) schizophrenics with a positi ve family history for schizophrenia-spectrum disorders, or (c) people in th e general population. Family-history-positive schizophrenics and their schi zophrenic relatives were, in turn, significantly less likely than their own non-schizophrenic biological relatives to be born in the winter; schizophr enics in these families tended to be born in the milder-weather seasons, pa rticularly the spring and fall. Results suggest that environmental factors associated with winter birth may be etiologically important in schizophreni a, particularly for cases in which familial liability factors are weak. By contrast, a familial, probably genetic, liability factor may be especially important in schizophrenics born in mild weather. (C) 2000 Elsevier Science B.V. All rights reserved.