Twin Concordance for Schizophrenia Is Not Entirely Hereditary

Monozygotic (MZ) twins have a much higher concordance rate for schizophrenia than dizygotic twins, even when the twins are raised separately. Some researchers interpret this as strong evidence of a genetic component for schizophrenia. But, an important difference between pairs of MZ twins may cast some doubt on this assumption: about 60% of MZ twin pairs share the same placenta and chorion (monochorionic) and, therefore, are more likely to be exposed to the same intrauterine factors than MZ twin pairs who do not share a placenta (dichorionic). This comparison of monochorionic and dichorionic MZ twin pairs shows that the two types of MZ twins have very different concordance rates for schizophrenia.

Using an analysis of fingerprint patterns to…