Homosexuality And The Older Brother Effect

Some of my ancestors floating in the Great Salt Lake around 1926. The guy in the back was a family friend. People use to take the train out to the Great Salt Lake for a day at the Saltair resort, therefore the matching swimsuits. No implication of anyones sexuality implied, I just enjoyed the photo of brothers.

As the oldest child this doesn’t apply to me, but the more older brothers a boy has, the higher the chance a boy will be gay. Younger brothers and sisters of any age have no correlation.

We don’t know why, but there’s an interesting theory. When a mother has a series of boys, and becomes pregnant with another male, her body may have an immune system response that affects fetal development, including sexual differentiation of the brain. This could be a reaction of her female body reacting to a series of male embryos or it could be an evolutionary adaptation to ensure there is a feminized boy in the family if girls fail to appear.

A later study validated the results, and added the calculation that each older brother increases the chances the next son is homosexual by 33%. At an average rate of homosexual orientation of 4%, it would take 9-10 older brothers to reach a 50-50 chance of being gay.


  • Gay Men: The more older brothers, the more likely a son is gay.
  • Lesbians: No sibling effect observed.


  • R. Blanchard and A.F. Bogaert, Proportion of homosexual men who owe their sexual orientation to fraternal birth order: an estimate based on two national probability samples, Am. J. Hum. Biol. 16 (2004), pp. 151–157.
  • R. Blanchard, Quantitative and theoretical analyses of the relation between older brothers and homosexuality in men, J. Theor. Biol. 230 (2004), pp. 173–187.


  1. Preston, Good to hear from u. I love that picture as well. The tall man in the back is Mom’s brother, Gilbert. The fellow in back of Mom is an unknown date. The gal next to Mom. Is Gil’s wife, Helen. In back of her is James. Henry and Walter are in front of Gill. The picture is amazing and historic. Thanks for reminding me of it again. Love, Aunt Alice

  2. Can you tell me where you got your picture? I’d like to use it and blow it up, if that’s be ok with you? Thanks!! It’s art for my spa.

  3. I heard a theory (can’t for the life of me remember where) that suggested with each progressive male birth, Testosterone levels dropped as Androgen became less effective on the fetus. As there are other factors that decrease testosterone levels in a developing fetus, such as stress in the pregnant mother, this accounts for older siblings also possibly becoming homosexual. Of course, it only generates a predisposition not a dyed-in-the-wool result.

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