Studying Schizophrenia: Genetic Factors

Genetic Factors and Schizophrenia


Genetic Factors: It has been known for a long period of time that schizophrenia runs in families. This could be explained by families sharing the same disadvantaged environment. However, research evidence suggests that genetic factors are important. Such evidence comes from three major sources:

• family studies,
• twin studies,
• and, adoption studies.

Evidence from family studies

First-degree relatives share (on average) 50% of their genes, and second-degree relatives share 25%. Studies compare rates of schizophrenia in relatives of diagnosed cases compared to relatives of controls to investigate the genetic transmission of schizophrenia. Now there is considerable amounts of evidence that implies that the closer the biological relationship, the greater the risk of developing schizophrenia (or a related psychotic disorder).

Evidence from twin studies

Twin studies offer another way of establishing genetic links, this is done by comparing the difference in concordance rates for identical (mono-zygotic) and fraternal (di-zygotic) twins. Both share the same environment, but only MZ twins have identical genetic make-up. Multiple studies have been conducted and all show a higher concordance rate in MZ twins than in DZ twins. To distinguish genetics conclusively from the environment, researchers have searched out MZ twins reared apart where at least one twin has been diagnosed with schizophrenia. Obviously, in this case there are few in number and an added problem is that one of the reasons for separation may have been a problem in the family. However, even in the rare cases where MZ twins are reared apart, they still share the same environment in their mother womb, therefore the contribution of environmental factors can't be entirely discounted.


Gottesman and Shields (1982) used the Maudsley twin register and found that 58% of MZ twin pairs were concordant for schizophrenia. If the genetic hypothesis is correct, then the children of a non-affected dischordant MZ twin should still be at high risk.

Evidence from adoption studies

A more competent way of separating out the effects of environment factors is to analyse adopted children who later develop schizophrenia and compare them with their biological and adoptive parents.
The Finnish Adoption study (that began in 1969 by Tienari) had identified 'adopted away' offspring of biological mothers who had been diagnosed with schizophrenia, plus a matched control group of 135 'adopted away' offspring of mothers who had not been diagnosed with any mentla disorder. The study reported that 7% of the index adoptees developed schizophrenia, compared to 1.5% of the control group.
The Danish Adoption study (Kety et al, 1994) took a national sample from across Denmark. The results where that high rates of diagnosis for chronic schizophrenia in adoptees who biological parents had the same diagnosis, on the contrary of the adoptee being adopted by 'healthy' parents.


From these case studies, it is indicated that there is a strong genetic link for schizophrenia. A criticism for these longitudinal studies, however, is that diagnostic criteria for schizophrenia is constantly being changed and updated.