The genetic link to schizophrenia is studied through three methods; family, twin and adoption studies.
Family studies aim to find a link between relatives. If there is a genetic link, it would be expected that closer relatives would be more likely to share schizophrenia compared to other members of the population.
Kendler Et Al found that first degree relatives of schizophrenics were 18 times more likely to also develop the disorder compared to others.
Twin studies aim to find whether monozygotic twins have a higher concordance rate compared to dizygotic twins, as MZ twins share 100% of their genetics whereas DZ twins share only 50%.
Joseph found a schizophrenia concordance rate of 40% for MZ twins and 12% for DZ twins. However it must be noted that identical twins are more likely to be treated as the same person due to more intense similarities and so environmental influences cannot be fully ignored.
Tienari et al found that 6.7% of adopted children who had schizophrenic mothers also developed the disorder, compared to 2% of those whos biological mothers were not schizophrenic. However it has been pointed out that selection bias may have influenced the adoptions.
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