Fraternal Birth Order and the Maternal Immune Hypothesis of Male Homosexuality

@article{Blanchard2001FraternalBO,
  title={Fraternal Birth Order and the Maternal Immune Hypothesis of Male Homosexuality},
  author={Ray Blanchard},
  journal={Hormones and Behavior},
  year={2001},
  volume={40},
  pages={105-114}
}
  • R. Blanchard
  • Published 1 September 2001
  • Biology
  • Hormones and Behavior
In men, sexual orientation correlates with an individual's number of older brothers, each additional older brother increasing the odds of homosexuality by approximately 33%. It has been hypothesized that this fraternal birth order effect reflects the progressive immunization of some mothers to Y-linked minor histocompatibility antigens (H-Y antigens) by each succeeding male fetus and the concomitantly increasing effects of such maternal immunization on the future sexual orientation of each… 

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  • Psychology
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  • 2006
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Sexual orientation, handedness, sex ratio and fetomaternal tolerance-rejection.

A new explanation for sexual relationships, sexual orientation, handedness and sibling SR is proposed, where lesbian embryos could induce tolerance from mothers with anti-female factors, and non-right-handedness could induce maternal tolerance, or change the maternal compatibility of "gay" embryos.
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Results are consistent with the hypothesis that the high birth order of homosexual men reflects the progressive immunization of certain mothers to H-Y antigen by succeeding male fetuses, and the increasing effects of H- Y antibodies on sexual differentiation of the brain in succeeding male Fetuses.

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How Many Gay Men Owe Their Sexual Orientation to Fraternal Birth Order?

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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.

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Homosexual men tend to be later-born children. Slater's index, the ratio of older sibs to all sibs, is consistently higher for male homosexuals than for comparable heterosexuals. According to some
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