Fraternal Birth Order Effect: The Science of Siblings and Sexuality

The Science of Siblings is a new series exploring how siblings can influence us, including our sexuality. Research shows a correlation between having older brothers and being gay, a phenomenon known as the “fraternal birth order effect.” Studies have consistently found that for every older brother a person has, their likelihood of being gay increases by about 33%. This effect has been documented worldwide, suggesting a potential biological mechanism.

One hypothesis is the “maternal immune hypothesis,” which suggests that a mother’s immune response to male-specific proteins produced by the Y chromosome of male fetuses may affect the development of subsequent male fetuses. However, this hypothesis doesn’t fully explain why the effect is also seen in women with older brothers.

Photo by Phil Nguyen

While the statistical findings are intriguing, they raise concerns about oversimplifying sexuality and potentially fueling harmful ideas. The first research on this topic did indeed begin in the 1940s and ’50s, during that era of investigations into what causes homosexuality, to be able to cure it. Justin Torres, author of “We the Animals,” expresses concern about the darker implications of these studies, such as the idea of “blaming” mothers or the notion that being gay in the womb is the only version of gayness that is real.

Despite the complexities and potential pitfalls of studying sexuality, researchers are drawn to understanding its mysterious nature. However, Torres, a humanist, questions the need for such explanations, preferring to view sexuality as a complex and joyous aspect of human experience that transcends simple explanations.

Why are we gay? Is it nature, nurture, genetics, rearing, how your daddy treated you or wasn’t there? There are so many different beliefs.

It was about 8 years ago when a TEDx video by Dr. James O’Keefe MD proposed a little different slant on the idea, but still very similar. You need to listen to his thoughts and see how they correlate to what was mentioned above. 


While listening to and watching this TEDx Talk, did your head nod, did it raise some questions about a couple of things, did you agree, tear up a little? The video may hit home for some of us.

We would love to hear what you think on the topic. Share your thoughts in the comments here or on Facebook.

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