title: Birth order and ratio of brothers to sisters in transsexuals.
abstract: BACKGROUND
  As previous studies with homosexual males have revealed a later birth order, more older brothers and more brothers than sisters, this research was extended to a large series of transsexual males and females, some of whom are homosexual.
  
  
  METHODS
  The male sample comprised 442 male-to-female transsexuals, subdivided by sexual partner preference: 106 homosexual, 135 heterosexual, 155 bisexual and 46 asexual. One hundred female-to-male transsexuals were also studied: 75 homosexual, 16 bisexual, seven heterosexual and five asexual. Birth order was computed by both Slater's Index and Berglin's Index.
  
  
  RESULTS
  Homosexual male-to-female transsexuals have a later than expected birth order and more older brothers than other subgroups of male-to-female transsexuals. Each older brother increases the odds that a male transsexual is homosexual by 40 %.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Psychiatry, Imperial College, School of Medicine, Charing Cross Hospital, London.
authors.forename: R
authors.initials: R
authors.lastname: Green
mesh_terms: D000328:Adult; D000728:Androgens / Q000097:blood; D001722:Birth Order; D005260:Female; D005333:Fetus / Q000378:metabolism; D005783:Gender Identity; D006182:H-Y Antigen / Q000097:blood; D018452:Homosexuality, Female / Q000235:genetics*; D018451:Homosexuality, Male / Q000235:genetics*; D006801:Humans; D016015:Logistic Models; D008131:London; D008297:Male; D008431:Maternal-Fetal Exchange / Q000276:immunology*; D009679:Nuclear Family; D011159:Population Surveillance; D011247:Pregnancy; D012306:Risk; D017678:Sex Distribution; D013739:Testosterone / Q000097:blood; D014189:Transsexualism / Q000235:genetics* / Q000276:immunology / Q000523:psychology
chemical_list: D000728:Androgens; D006182:H-Y Antigen; D013739:Testosterone