From the data on 57 co-twins of schizophrenics available at the MRC Psychiatric Genetics Research Unit, the morbidity rate of the monozygotic co-twins was obtained, using a modified Weinberg method, as a function of time elapsed after the onset in the twin who first became ill (referred to as "the first twin") and examined in relationship to modifying exogenous factors.
(1) The morbidity was found to be an exponential curve N = 70e-0.0424t, where N is the percentage of co-twins still remaining well t-years after the first twin's onset, except in the initial two years where the morbidity is significantly higher and deviates from this formula. A similar exponential curve also fitted Mitsuda's schizophrenic twin data.
(2) This "initial high morbidity" was much more marked in the second twins who were living together with the first twin at the time of the first twin's onset, suggesting an environmental factor as being responsible for this deviation.
(3) There was no support for the hypothesis that the closeness of the age of onset in this "living together" group is entirely due to the fact that they happened to be living together when an exogenous factor influential enough to precipitate the onset in both, occurred.
(4) The onset-admission interval was longer in twins who were living together and where the second twin became ill within two years, but the difference was not statistically significant. However, the exposure was found to have occurred more frequently just before or during the onset of the second twin, a statistically significant finding suggesting the significance of exposure to the first twin's influence.
(5) An estimation of morbidity risk was obtained and was found to be 75.7 per cent.