Short Wave Sibling order may affect sexuality and identity
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Mar 10, 2026 
Scott Semenina, a psychology professor who explains the fraternal birth order effect, and Justin Torres, a novelist and essayist reflecting on queer identity. They discuss how older brothers link to male same-sex attraction, the maternal immune hypothesis, surprising large-scale findings, and cultural risks of research into sexuality.
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Fraternal Birth Order Effect Is Global And Consistent
- The fraternal birth order effect is a consistent statistical pattern where men with more older brothers are more likely to be gay.
- Studies across countries (U.S., Turkey, Brazil, Netherlands, Samoa, Mexico) repeatedly find younger brothers are overrepresented among gay men.
Each Older Brother Raises Odds By A Third
- Each additional older brother raises a man's chance of same-sex attraction by about 33 percent per brother.
- That increases baseline odds from ~2% to about 2.6% with one older brother and ~8% with five older brothers.
Maternal Immune Hypothesis Offers Biological Mechanism
- The leading biological explanation is the maternal immune hypothesis: mothers develop antibodies to male-specific fetal proteins that affect later male fetal brain development.
- A 2017 study found mothers of gay sons had higher levels of these antibodies, supporting plausibility.