Acta Psychiatr Scand zyxwvutsrqpon 1997: 95: 1-5 Printed zyxwvutsrqpon in UK zyxwvutsrqpon - all rights reserved Copyright 0 Munksgaard 1997 zy ACTA PSKCHIATRICA SCANDINAVICA ISSN OOOI-69OX Review article A zyxw critical review of genetic studies of schizophrenia. I. Epidemiological and brain studies Portin P, Alanen YO. A critical review of genetic studies of schizophrenia. I. Epidemiological and brain studies. Acta Psychiatr Scand 1997: 95: 1-5. zyxwvu 0 Munksgaard 1997. Family, twin and adoption studies suggest that susceptibility zyxwvu to schizophrenia is based at least to some extent on genes. The results of these studies vary and depend greatly on the diagnostic criteria used. When a broad definition of schizophrenia is employed, even high degrees of heritability are observed. However, when more narrow criteria are employed, the degrees of heritability decrease. According to a large Finnish adoption study, the patient-family interaction has an even more important role in the origin of schizophrenia than purely genetic factors. Interaction between the genetic factors on the one hand and physical, psychological and psychosocial factors on the other hand is emphasized. zyxwvut Introduction While the average lifetime prevalence of schizo- phrenia in the adult population, regardless of race or country, is about 1%, in the siblings of patients it is 8-lo%, and in the children of patients it is 12- 15% (1). This indicates that there is a genetic component in the aetiology of schizophrenia, but is insufficient evidence to confirm that schizophre- nia is a simple, genetically transmitted disease. The aetiology of the disorder may also be of a more complex nature, including the effects of physical and psychosocial environmental factors. If we assume that environmental factors also play a role, restricted genetic hypotheses lead - as was pointed out 40 years ago by Planansky (2) - to an analysis based on a single variable in a field actually governed by two or more variables. Research in this field is 'also hampered by the fact that many geneticists have a poor understanding of the con- fusing facets of mental disorders, and many clinical psychiatrists have no access to the conceptually and technically demanding field of genetics (3). More specific epidemiological information about the genetics of schizophrenia can be obtained from twin and adoption studies. However, twin studies do not permit reliable discrimination between hereditary and environmental factors, since P. Portin'. 1 0. Alanen 'Laboratory of Genetics, Department of Biology and 'Department of Psychiatly, Institute of Clinical Medicine, University of Turku. Turku, Finland Key words: schizophrenia; gene; locus; twin studies; adoption; susceptibility Petter Portin. Laboratory of Genetics, Department of Biology, University of Turku, FIN-ZOO14 Turku. Finland Accepted for publication March 16, 1996 monozygotic twin pairs not only have the same genes but also share the same growing environ- ment. The number of monozygotic twin pairs reared apart, one or both of whom have become schizophrenic, is too small to provide an answer to this question. With regard to adoption studies, the only study conducted with both a genetic and psychosocial orientation and methodology has con- firmed the significance of both genetic and envi- ronmental factors (4-6). The modern molecular-genetic studies reviewed in Part I1 of this study (7) also appear to show that simple genetic hypotheses are not valid where schizophrenia is concerned. Twin studies According to twin studies, the concordance for monozygotic twins varies between 15 and zy 58% and that for dizygotic twins ranges between 4 and 27%, depending in part on the diagnostic criteria used Recent twin studies of schizophrenia have involved the offspring of discordant monozygotic twins. Gottesman and Bertelsen (20) found no difference between the prevalence of schizophrenia (8-19). 1