Fraternal birth order linked to same-sex attraction in men

Several studies have found fraternal birth order to be a factor that influences the odds of a man being gay.

Gay men tend to have a greater number of older brothers compared to straight men, straight women, and gay women. This by no way means that a large number of older brothers guarantees that a boy will grow up to be gay—or that boys without older brothers will not be gay. Rather, having older brothers increases the odds of a man being gay. For instance from a 2% chance (no older brothers) to a 6% chance (4 older brothers).

How significant is the birth-order effect?

According to J. Michael Bailey, PhD (2016), and fellow researchers, the fraternal-birth-order effect is significant, “in the sense that it increases the odds of a man’s homosexuality appreciably. Assuming that a man without any older brothers has a 2% chance of being homosexual, a man with one older brother has a 2.6% chance; with two, three, and four older brothers, the chances are 3.5%, 4.6%, and 6.0%, respectively.” (Bailey et al., 2016 p. 79).

What about non-biological brothers?

Anthony F. Bogaert, PhD, has demonstrated that the fraternal-birth-order effect is only relevant for biological, as opposed to non-biological (such as adopted) older brothers, suggesting that it is prenatal in origin: “Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men's sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.” (Bogaert, 2006, Abstract).

Why would fraternal birth order influence the odds that a man would be gay?

Scientists cannot provide a definitive answer, but Ray Blanchard, Ph.D. (2001), Professor of Psychology at the University of Toronto, has developed an intriguing theory.

Dr. Blanchard explains:

“It has been hypothesized that the 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 succeeding male fetus.”

Dr. Blanchard has hypothesized that some mothers’ bodies develop antibodies in reaction to a male fetus, with the antibodies increasing with each successive (male) pregnancy. According to Dr. Blanchard’s hypothesis, these antibodies have the capacity to influence sexual differentiation in the fetal brain, leading to an increased incidence of same-sex attraction:

“[A]nti-H-Y antibodies produced by the mother pass through the placental barrier to the fetus and affect aspects of sexual differentiation in the fetal brain.” (Blanchard, 2001, Abstract)

While Dr. Blanchard’s hypothesis is just that—a hypothesis—it has received a great deal of attention in the scientific community because it is consistent with a variety of evidence.

Dr. Blanchard notes:

“This explanation is consistent with a variety of evidence, including the apparent irrelevance of older sisters to the sexual orientation of later born males, the probable involvement of H-Y antigen in the development of sex-typical traits, and the detrimental effects of immunization of female mice to H-Y antigen on the reproductive performance of subsequent male offspring. The maternal immune hypothesis might also explain the recent finding that heterosexual males with older brothers weigh less at birth than heterosexual males with older sisters and homosexual males with older brothers weigh even less than heterosexual males with older brothers.” (Blanchard, 2001, Abstract)


Bailey, J.M., Vasey, P.L., Diamond, L.M., Breedlove, S.M., Vilan, E. & Epprecht, M. (2016). Sexual orientation, controversy, and science. Psychological Science in the Public Interest, 17(2), 45-101.

Blanchard, R. (2001). Fraternal birth order and the maternal immune hypothesis of male homosexuality. Hormones and Behavior, 40(2), 105-114.

Bogaert, A.F. (2006). Biological versus nonbiological older brothers and men's sexual orientation. Proceedings of the National Academy of Sciences of the United States of America, 103(28), 10771-10774. doi:10.1073/pnas.0511152103

Gender Identity Gender identity icon Our core sense of who we are as a man, a woman, a mixture of both, or neither.

Gender Expression Gender expression icon How we show up in the world through choices like clothing, hair style, mannerisms or tone of voice.

Attraction attraction icon How we feel toward others sexually, romantically and/or emotionally.

Biological Sex Biological sex icon Physical attributes such as reproductive organs and genitalia, chromosomes, genes and hormone levels.

Explore More

Read the full research article by Dr. Anthony F. Bogaert referenced in this article.

Biological versus nonbiological older brothers and men’s sexual orientation

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Read the summary of the research by Dr. Ray Blanchard referenced in this article.

Fraternal birth order and the maternal immune hypothesis of male homosexuality

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