A series of 17 monozygotic twins discordant for schizophrenia was studied intensively at the National Institute of Mental Health between 1963 and 1967. Sixteen pairs were interviewed a mean of 7.8 years after original admission.
It was found that one pair is now concordant for schizophrenia, three previously healthy co-twins show new psychopathology of a possibly schizophrenic nature, and 12 twin pairs remain stably discordant.
Robert Belmaker, William Pollin, Richard Jed Wyatt, Stephen Cohen. A Follow-Up of Monozygotic Twins Discordant for Schizophrenia. Arch Gen Psychiatry. 1974;30(2):219–222. doi:10.1001/archpsyc.1974.01760080073012