The observation that schizophrenia is more commonly observed among the rela
tives of individuals with schizophrenia than in the general population does
not indicate the mechanism that produces such familiality occurs. Adoption
designs permit evaluation of the role of genetic factors in schizophrenia
independently of the influence of family environments. Results from studies
of adoptees with schizophrenia and their biological and adoptive relatives
indicate that genetic factors play a highly significant role in the risk f
or schizophrenia. This genetically mediated risk to relatives includes an i
ncreased prevalence of both schizophrenia and a nonpsychotic syndrome analo
gous to schizophrenia, but does not represent a general liability to other
forms of psychopathology. Although adoption studies have convincingly demon
strated an important role for genetic factors in schizophrenia, the necessi
ty and specificity of such factors, their precise identity, and their inter
action with environmental influences remain unknown. (C) 2000 Wiley-Liss, I
nc.