The incidence and prevalence of transsexualism are difficult to establish, as the syndrome is not sufficiently common to lend itself to the usual methods of epidemiological research.

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A pair of male monozygous twins of gynandromorphic habitus but normal male body hair and genitalia, who appear concordant for transsexualism and explosive personality disorder but discordant for schizophrenia, is presented. No evidence of specific family dynamic factors, imitative learning or folie a deux was found in the twins' history. A constitutional aetiology of their transsexualism and one twin's schizophrenia is suggested. The relationship between transsexualism and schizophrenia is briefly reviewed. No evidence can be found that the modal transsexual is suffering from a schizophrenic disorder, florid or attenuated. A more subtle relationship is possible of transsexualism occurring as a schizophrenic spectrum disorder. This twin pair appears to illustrate such a relationship.

The syndrome of transsexualism has become widely known in psychiatry and is now clearly distinguished from both homosexuality and transvestism, as well as from hermaphrodism (Benjamin, 1953). Other authors such as Wålinder (1968), Hoenig et al. (1964) and others have given definitions of the syndrome, which might be circumscribed as follows: The transsexualists, although physically apparently normal, are nevertheless convinced that they are ‘inwardly really’ persons of the opposite sex; they claim to ‘feel’ like a person of the opposite sex, but think that the body has somehow—by a freak of nature—developed abnormally, and not in accordance with their ‘true nature’. They seek medical treatment, if necessary surgery, to have their body changed into what they conceive to be their ‘real’ sex.

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