Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review

@article{Bogaert2011SexualOF,
  title={Sexual orientation, fraternal birth order, and the maternal immune hypothesis: A review},
  author={Anthony F Bogaert and Malvina N Skorska},
  journal={Frontiers in Neuroendocrinology},
  year={2011},
  volume={32},
  pages={247-254}
}
In 1996, psychologists Ray Blanchard and Anthony Bogaert found evidence that gay men have a greater number of older brothers than do heterosexual men. This "fraternal birth order" (FBO) effect has been replicated numerous times, including in non-Western samples. More recently, strong evidence has been found that the FBO effect is of prenatal origin. Although there is no direct support for the exact prenatal mechanism, the most plausible explanation may be immunological in origin, i.e., a mother… 
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