Short Wave Podcast Summary

Episode: "Sibling order may affect sexuality and identity"

Host: Selena Simmons-Duffin (NPR)
Guests: Justin Torres (writer, National Book Award winner), Jan Kabatek (social scientist, University of Melbourne), Scott Semenina (psychology professor, Stetson University)
Date: March 10, 2026

Main Theme

This episode unpacks scientific research around the "fraternal birth order effect"—the observed correlation between sibling order, specifically the presence of older brothers, and the probability of a person being gay. Through expert interviews and personal reflections, the episode explores historic biases in the study of sexuality, delves into modern biological hypotheses, and raises ethical considerations about seeking biological explanations for queerness.

Key Discussion Points & Insights

1. Origins of Scientific Curiosity in Sexuality (00:21–02:17)

• Historic context:
  • Early scientific studies into queer identity emerged in the mid-20th century, influenced by eugenics and often driven by harmful intentions such as "treating" or "curing" homosexuality.
  • Justin Torres (01:04):

    "There's this sudden visibility of underground queer culture. And then the concern is that there's something pathological happening with these people."

  • Queer identities were criminalized and living openly was extremely dangerous.
• Justin Torres (01:53):

  "It was criminal, it was career destroying, life destroying... the closet was a dangerous place to be. Outside of the closet was a dangerous place to be."

2. Sibling Order and Sexual Orientation: What We Know (02:17–04:13, 05:21–06:36)

• Limited understanding:
  • Despite decades of research, scientists have only scratched the surface of what shapes sexual orientation.
  • Jan Kabatek (02:34):

    "We still have very little idea about what underlies the origins of sexual orientation."

• Fraternal birth order effect:
  • Consistent finding that gay men are statistically more likely to have multiple older brothers.
  • This finding has been observed across various cultures and countries (US, Turkey, Canada, Netherlands, Samoa, Mexico, Brazil, etc.).

Notable quote — Jan Kabatek (03:37):

"It's basically established as kind of a truth, even though we have to be very careful with the term truth when it comes to science."

3. The Science Behind the Statistic (05:21–09:15)

• Magnitude of effect (Scott Semenina):

  • Baseline chance of being gay: ~2%
  • Each older brother increases a man's odds of being gay by about 33%.
    • Example: One older brother → 2.6%; two older brothers → 3.6%; five older brothers → about 8%.
  • Scott Semenina (05:44):

    "The fraternal birth order effect shows about a 33% increase in the probability of like male same sex attraction. For every older brother that you have."

• Quantifying the impact:

  • Younger brothers with multiple older siblings are statistically more likely to enter same-sex marriages compared to the eldest.
    • Jan Kabatek (06:36):

      "The probabilities are about 80% greater for the man who is the youngest child with three older brothers compared to the eldest child with three younger brothers."

  • This effect is consistent but not deterministic; many gay people have no brothers and vice versa.

• Leading explanations:

  • Maternal immune hypothesis:
    • Mother's immune response to male fetal proteins grows stronger with each successive male pregnancy, potentially influencing sexual orientation.
    • Scott Semenina (07:28):

      "When a male fetus is developing... the mother's immune system forms somewhat of an immune response to those proteins."

    • In 2017, studies found mothers of gay sons had more antibodies targeting male-specific proteins.

4. New Research & Surprising Findings (08:18–09:15)

• Jan Kabatek’s massive study:
  • Research involving over 9 million people found the fraternal birth order effect present, but with a twist:
    • Women in same-sex marriages were also more likely to have older brothers.
    • Jan Kabatek (08:35):

      "We also showed that the same association manifests for women."

  • This challenges the maternal immune hypothesis, which was presumed to operate only in males.
  • The true mechanism remains unresolved; there may be other biological or social influences.

5. Human Implications & Complexities (10:06–12:32)

• Justin Torres' personal perspective:

  • Fits the pattern (two older brothers), but is wary of searching too hard for biological justifications.
  • Recalls a disturbing moment from his youth about public attitudes towards gay children:
    • Justin Torres (10:44):

      "I remember... the host polled the audience and said, if there was a test and you could know if your child was gay, would you abort? ... just being so horrified and disturbed, watching all those hands go up ... and just feeling so hated."

  • Finds the statistical correlation both funny and worrying; sees fun in the pattern's ubiquity but danger in letting scientific inquiry replace human complexity.
    • Justin Torres (12:01):

      "It's 45 things at once. Yeah. I mean, I love birth order in general. It also doesn't really determine who we are in the world. I do think it's fun. I think it's worrisome when the science takes itself too seriously."

    • Justin Torres (12:18):

      "As a kind of humanist, I just don't know why we need to look for explanations for something as complex and joyous and weird as sexuality."

• Selena Simmons-Duffin wraps up:

  • Highlights that the beauty, complexity, and joy of queer identity will always extend far beyond statistics and scientific models.

Memorable Moments & Quotes

• "The closet was a dangerous place to be. Outside of the closet was a dangerous place to be."
  — Justin Torres (01:53)
• "It's basically established as kind of a truth, even though we have to be very careful with the term truth when it comes to science."
  — Jan Kabatek (03:37)
• "Each older brother increases the probability of male same sex attraction by about 33%."
  — Scott Semenina (05:44)
• "The probabilities are about 80% greater for the man who is the youngest child with three older brothers..."
  — Jan Kabatek (06:36)
• "It’s 45 things at once. ... I think it’s worrisome when the science takes itself too seriously."
  — Justin Torres (12:01)
• "I just don’t know why we need to look for explanations for something as complex and joyous and weird as sexuality."
  — Justin Torres (12:18)

Timestamps of Key Segments

• 00:21–02:17: History of queer research and its perils (Justin Torres)
• 03:05: Fraternal birth order effect described (Jan Kabatek)
• 05:21–06:36: Quantifying the effect and real-world implications (Scott Semenina, Jan Kabatek)
• 07:28–07:53: Maternal immune hypothesis explained (Scott Semenina)
• 08:35–09:15: New findings about women and the effect's universality (Jan Kabatek)
• 10:06–12:32: Justin Torres’ reflections and concerns on biological determinism vs. lived experience

Notable Public Figures (Fun Fact Section) (13:31–13:46)

• Host Selena notes her rabbit hole research:
  • Abby Wambach is the youngest of seven with four older brothers.
  • Nathan Lane is the youngest of four.

Tone:
Warm, curious, thoughtful, and occasionally humorous but always respectful when addressing sensitive or complex topics.

Useful For:
Anyone interested in the intersection of science and LGBTQ+ identity, the ethics of studying sexuality, or science journalism that wrestles with nuance and humanity.