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There are many published twin studies of schizophrenia. Although these studies have been reviewed previously, to our knowledge, no review has provided quantitative summary estimates of the impact of genes and environment on liability to schizophrenia that also accounted for the different ascertainment strategies used.

To calculate meta-analytic estimates of heritability in liability and shared and individual-specific environmental effects from the pooled twin data.

We used a structured literature search to identify all published twin studies of schizophrenia, including MEDLINE, dissertation, and books-in-print searches.

Of the 14 identified studies, 12 met the minimal inclusion criteria of systematic ascertainment.

By using a multigroup twin model, we found evidence for substantial additive genetic effects—the point estimate of heritability in liability to …