I don&#x27;t know that off-hand, but the book I cited may provide enough of the raw data to suggest a magnitude. By memory, I venture to say that more than 10 percent of monozygotic twins of schizophrenia cases (probands) do not have a schizophrenia diagnosis of their own--but I&#x27;ll revise this figure if someone can find a good reference. What&#x27;s clear in general about the research is that schizophrenia is a polygenic condition (influenced by many genes) and multifactorial (also influenced by factors other than genes).