Mental Illness, Genetics of
Only adoption studies can provide solid evidence for the existence of heritable factors. Twin studies, showing that the concordance rate in monozygotic twins exceeds that in dizygotic twins, are weakened because monozygotic twins are likely to be treated more similarly by significant persons in their environment, and may also tend to identify psychologically with each other. Studies of family process, showing that families with disturbed communication patterns are more likely to have a schizophrenic child, are not compelling evidence for nongenetic factors, because the disturbed behavior of the parents could be a consequence of the behavior of the child, or a parallel effect of the same genes that caused schizophrenia in the offspring. There are, however, adoption studies in both schizophrenia and affective disorder that point toward the existence of heritable factors.
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