What is it about?

This article adds to the ongoing scientific discussion about why some males are exclusively same-sex attracted. It is consistently found that these males have more older brothers than straight men, called the Fraternal Birth Order Effect. The leading explanation is that a mother's immune system reacts to male pregnancies, and this immune reaction can influence later sons' fetal brain development and sexual orientation. We find evidence for this birth order effect but did not find evidence that mothers of same-sex attracted males reproduce more, a reproductive effect (of mothers) proposed to explain how genes for male same-sex attraction are maintained across time. Other studies come to similar conclusions, but this study uses a sample from Independent Samoa, where women tend to have more children than in the West. This makes it a good place to study evolutionary questions. Even more interesting, Samoans recognize more gender categories than just men and women. If you are a male in Samoa who is same-sex attracted, you tend to be rather feminine and identify as a fa’afafine—a gender category distinct from men and women.

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Why is it important?

This study makes us more certain that birth order genuinely impacts male sexual orientation, although this was well-established. If mothers of same-sex attracted males reproduced more, it might offset the fact that their sons’ attractions have a partial genetic basis, but reduced reproduction. We didn’t find evidence that this was true, so one prominent evolutionary explanation of male same-sex attraction needs to be re-examined. Some science clarifies our understanding, but this study also highlights complication. Birth order is a robust and reliable correlate of male same-sex attraction, but we’re less certain how genes for this trait can be maintained across evolutionary time. We need to refine our ideas, or go back to the drawing board entirely, then collect more data.


This study drew from an archive of Samoan data to look at the research questions in a new way. The results remind me to approach scientific explanations with humility and a willingness to change my mind. I've had the great fortune of good mentorship and years of productive research collaborations with Dr. Vasey and several of his students. Each of the co-authors was trained by Dr. Vasey, and we now hold faculty positions at various universities. We make a good team. This collaborative effort was a nice way to look back at older data, helping us look at research ahead with more clarity.

Scott Semenyna
Stetson University

Read the Original

This page is a summary of: Male androphilia, fraternal birth order, and female fecundity in Samoa: A 10-y retrospective, Proceedings of the National Academy of Sciences, December 2023, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2313284120.
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