Discordance for schizophrenia in monozygotic twins has been interpreted as evidence of the role of environment in the etiology fo schizophrenia, with "environment" usually, in practical parlance, carrying Freudian and Sinnerian connotations. MZ twins are often discordant for certain brain development deviations (i) to which they are specially prone, (ii) which show other evidence of having genetic etiologies, and (iii) which involve embryonic symmetry development. MZ twinning is an embryonic symmetry anomaly; its association with others is not surprising. Neurophysiological manifestations of schizoprenia are markedly asymmetric, as is much of normal human mental function. Unusual brain function asymmetry is herein shown to be a factor in virtually every appropriately tested case of MZ discordant schizophrenia.