Despite variations in concordance rates the fact that monozygotic twins are more often concordant for schizophrenia has been one of the major pieces of evidence supporting the genetic hypothesis of schizophrenia. While not denying the role of heredity in schizophrenia, this report concerns itself with several factors that frequently are not noted in these studies which have etiologic and methodologic implications. Specifically there are substantial physiologic and developmental differences evident in monozygotic twins which are well documented but rarely acknowledged in the existing twin studies. The role of these physiological and developmental differences may have pertinent theoretical implications for the varied findings noted in existing twin studies as well as schizophrenia in general.
Campion E, Tucker G. A Note on Twin Studies, Schizophrenia and Neurological Impairment. Arch Gen Psychiatry. 1973;29(4):460–464. doi:10.1001/archpsyc.1973.04200040016003