Overview of Sibling order may affect sexuality and identity
This Shortwave (NPR) episode, hosted by Selena Simmons-Duffin, explores the "fraternal birth order effect" — a repeatedly observed statistical pattern in which men attracted to men tend to have more older brothers than other men. The episode covers the history of the research, key findings and statistics, leading biological hypotheses (especially the maternal immune hypothesis), new large-sample results that complicate the story, and ethical concerns raised by queer writers and scholars.
Key findings — what the effect looks like
- Baseline probability used in the episode: ~2% chance a man is gay (rounded for illustration).
- Fraternal birth order effect: about a 33% relative increase in the probability of male same-sex attraction for each older biological brother.
- Example: 1 older brother → ~2.6% chance; 2 older brothers → ~3.6%; 5 older brothers → ~8%.
- The effect has been replicated across many countries (U.S., Turkey, Canada, Netherlands, Samoa, Mexico, Brazil, etc.).
- It is probabilistic, not deterministic: many gay men have no older brothers, and many people with multiple older brothers are straight.
How researchers explain it
- Maternal immune hypothesis (leading biological explanation):
- A pregnant mother develops antibodies in response to male-specific fetal proteins (from Y-chromosome products).
- With each subsequent male pregnancy, those maternal antibodies could increasingly influence sexual differentiation of the brain in later male fetuses, altering odds of same-sex attraction.
- Support cited: a 2017 study found higher levels of certain antibodies in mothers of gay sons versus mothers of heterosexual sons or mothers without sons.
New findings and complications
- Jan Kabatek et al. published a massive study (>9 million people) examining sibling order and same-sex marriage records.
- Confirmed that people in same-sex marriages were more likely to have older brothers — but also found the association for women in same-sex marriages.
- The presence of the effect in women challenges the simple version of the maternal immune hypothesis (which specifically predicts an effect for later-born males).
- Implications:
- The maternal immune hypothesis remains plausible but no longer as cleanly explanatory.
- Alternative explanations or broader sibling-influence mechanisms may play a role.
- More prospective and mechanistic research is needed.
Broader context and history
- Early sexology research (mid-20th century) was often entangled with eugenics and pathologizing aims; initial studies were conducted in a social context where being queer could be criminal or dangerous.
- This historical context raises ethical concerns about how biological findings about sexuality might be used or misused.
Voices and ethical concerns
- Justin Torres (author, queer, youngest of three) provides a personal reaction:
- Mixed feelings — intrigued and amused by pattern recognition, but deeply worried about potential harms (e.g., testing, stigmatization, choices like abortion if prenatal predictors were available).
- Emphasizes that human identity is complex, and scientific explanations can be reductive or weaponized.
- Researchers caution against overinterpreting the effect; it’s one statistical pattern among many influences on sexuality.
Main takeaways
- The fraternal birth order effect is a robust statistical finding: each older brother increases the relative odds of a man being gay by about one-third.
- The effect is neither all-encompassing nor explanatory of individual cases — it shifts probabilities, it doesn’t determine identity.
- Biological mechanisms (like the maternal immune hypothesis) are plausible but not conclusively proven, especially given recent large-sample findings that complicate male-specific predictions.
- Historical misuse and ethical risks mean scientific claims about sexuality must be communicated and applied carefully.
Notable quote
- Justin Torres: “I think that that danger is not subsided just because we have more tolerance... I think that that danger is not subsided just because we have more tolerance in this particular place in this moment in time.”
Credits: episode produced by Rachel Carlson; host Selena Simmons-Duffin; edited by Rebecca Ramirez.