In a study looking at classical schizophrenia, Kety and Ingraham*, identified adoptees with schizophrenia and then compared the prevalence of this condition in their genetic relatives and adopted relatives. They found the prevalence of schizophrenia to be ten times higher in the genetic relatives than in the adopted relatives, thereby suggesting that inherited genetic factors had a greater influence on the development of schizophrenia than environmental factors.
*Kety SS, Wender PH, Jacobsen B, Ingraham LJ, Jansson L, Faber B, Kinney DK.
Mental illness in the biological and adoptive relatives of schizophrenic adoptees. Replication of the Copenhagen Study in the rest of Denmark. Arch Gen Psychiatry. 1994 Jun;51(6):442-55.
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