Birth order and ratio of brothers to sisters in transsexuals

Abstract

Hypotheses explaining the extension of prior findings to this large sample of transsexual males include a progressive maternal immunization to the male foetus either through the H-Y antigen or protein-bound testosterone or alterations in foetal androgen levels in successive pregnancies, all modifying male psychosexual development. Data on the sexual orientation of younger brothers of homosexual male transsexuals in this study are not consistent with the progressive immunization hypothesis.