• schizophrenia;
  • gene;
  • locus;
  • twin studies;
  • adoption;
  • susceptibility

Family, twin and adoption studies suggest that susceptibility 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.