Sibling order may affect sexuality and identity

Summary

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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

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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.

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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."

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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."

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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.

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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.

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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.

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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)

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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

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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.

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

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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.

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Transcript

[NPR Sponsor Announcer]

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[Selena Simmons-Duffin]

Hi shortwavers. Selena Simmons Duffin in the host chair. Years ago, I can't remember exactly when I became aware that gay people are often the youngest kids in their families. As a gay person who's the youngest in my family, there was something kind of appealing about this idea. Like there was a statistical order to things, and I fit neatly into that order. When I started reporting on the science behind the idea, the whole thing turned out to be much more interesting than I originally imagined. Also stranger and darker. That darkness comes in part from how scientists first started researching what makes people queer in the first place. Near the middle of the last century,

[Justin Torres]

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

[Selena Simmons-Duffin]

That is writer Justin Torres. He's thought a lot about the way scientists have studied sexuality. Last year he won the National Book Award for a novel titled Blackouts.

[Justin Torres]

My novel is kind of interested in these kind of pre Kinsey sexology studies, specifically, specifically this one called Sex Variance. You know, it was really informed by eugenics and they were looking for the cause of homosexuality in the body in order to treat it or cure it or get rid of it.

[Selena Simmons-Duffin]

The queer people scientists were studying were also living in a world where this facet of their identity was dangerous.

[Justin Torres]

It was criminal, it was career destroying, life destroying. To be outed against your will was incredibly dangerous. And to live out was dangerous as well, because then of course you get backlash and you get persecution. So the closet was a dangerous place to be. Outside of the closet was a dangerous place to be.

[Selena Simmons-Duffin]

So researchers first began studying queer people for generally sinister reasons at a time when being queer was dangerous. And the studies themselves turned out to be really hard, says Jan Kibatek, a social scientist at the University of Melbourne.

[Jan Kabatek]

Most of it fell flat, meaning that we still have very little idea about what underlies the origins of sexual orientation.

[Selena Simmons-Duffin]

With an exception, the one thing that researchers zeroed in on that seemed to

[Jan Kabatek]

be actually real was men, specifically who exhibit attraction to the same sex are likely to have more older brothers than other types of siblings.

[Selena Simmons-Duffin]

So gay men had lots of older brothers. In the 1990s, this was dubbed the fraternal birth order effect. In the years since, this effect has been found again and again all over the world in the US And Turkey and Canada and the Netherlands and Samoa and Mexico and, and Brazil pretty much everywhere. It's been studied. Lots of gay men are the youngest brothers in their families.

[Jan Kabatek]

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.

[Selena Simmons-Duffin]

Today on the show the Fraternal Birth Order Effect, what it is, how it's been studied and, and what it can and can't explain about sexuality. You're listening to Short Wave, the science podcast from npr.

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[Selena Simmons-Duffin]

with the basics of how the fraternal birth order effect plays out. I asked Scott Semenina about this. He's a psychology professor at Stetson University. How likely is it at baseline that someone will be gay?

[Scott Semenina]

Somewhere around 2 to 3%. We can call it 2% just for the sake of simplicity.

[Selena Simmons-Duffin]

Okay. And so how big is the fraternal birth order effect?

[Scott Semenina]

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

[Selena Simmons-Duffin]

got one older brother like me, your chance of being gay nudges up to about 2.6%.

[Scott Semenina]

And then that probability would increase another 33%. If there was a second older brother,

[Selena Simmons-Duffin]

then you're looking at about 3.6%. And on and on. If you have five older brothers, your chance of being gay is about 8%. So it's not huge, but it is remarkably consistent across studies. And the odds can add up and translate to higher chances of younger brothers actually getting married to someone of the same sex. Here is Jan Ka Batek, the researcher in Melbourne, talking about his research.

[Jan Kabatek]

Let's consider two men who have three brothers. One man is the eldest child in the family and the other is the youngest. And so if we quantify the probabilities of entering same sex union, the probabilities are about 80% greater for the man who is the youngest child with three older brothers compared to the man who is the eldest child and has three younger brothers.

[Selena Simmons-Duffin]

Although of course all four brothers might well be straight. And certainly plenty of gay people have no brothers at all. This is not the only influence on a person's sexuality. Still, the consistency of this effect makes you wonder why would this be? Why would gay people tend to have lots of older brothers? Scott Simoneina explains There's been a leading theory to explain the maternal immune hypothesis.

[Scott Semenina]

The basic version of this hypothesis is that when a male fetus is developing, the Y chromosome of the male produces proteins that are going to be recognized as foreign by the mother's immune system and it forms somewhat of an immune response to those proteins.

[Selena Simmons-Duffin]

And that immune response has some effect on the development of subsequent male fetuses.

[Scott Semenina]

In 2017, the plausibility of this hypothesis was bolstered quite a bit by finding that mothers of gay sons have more of these antibodies that target these male specific proteins than mothers of sons who are not gay or mothers who have sons whatsoever.

[Selena Simmons-Duffin]

However, the plausibility of this hypothesis is newly up for debate. Jan Kabatek and colleagues published a study on the fraternal birth order effect with a huge sample, more than 9 million people. They found that people in same sex marriages had lots of older brothers.

[Jan Kabatek]

With a twist, interestingly enough, and this is quite different from what has been done before, we also showed that the same association manifests for women.

[Selena Simmons-Duffin]

In other words, women who were in same sex marriages were also more likely to have older brothers than other types of siblings. That really surprised a lot of people in the field and kind of threw into question the maternal immune hypothesis.

[Jan Kabatek]

One option is that the hypothesis works for both men and women. Of course there can be also other explanations and we still have not really good idea what kind of explanations these could be. But it's for the prospective research to make this clearer.

[Selena Simmons-Duffin]

Is there another way to think about this besides statistical oddity? Is there something more profound or deeper to think about it?

[Jan Kabatek]

I don't think of this as just a statistical oddity. It actually fits quite well into a broader realm of research which deals with sibling influences. But just the fact that we are observing effects that are so strong, relatively speaking, implies that there's a good chance that there's, at least partially, some biological mechanism that is driving these associations. Whether this mechanism is the maternal immune hypothesis is an open question, but I wouldn't be completely dismissive of that possibility.

[Selena Simmons-Duffin]

Okay, so how does all of this sound to Justin Torres, a queer person who actually fits this pattern?

[Justin Torres]

I grew up with two older brothers. It was just the three of us, and it was pretty wild. There was a lot of kind of masculine energy.

[Selena Simmons-Duffin]

Okay, so, Justin, in one way, I'm like, this is kind of fun. Like, it's concrete, and it's everywhere, all over the world that this thing happens. And there's this theory as to why it might be happening. But then I feel like the reason why I wanted to talk to you is to kind of wrap my arms around the other undercurrents.

[Justin Torres]

It is the undercurrents that worry me immensely. I remember when I was a kid, I have this memory of watching daytime television, and 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? And I remember just being so horrified and disturbed, watching all those hands go up in the audience, you know, and just feeling so hated, you know? And, like, at that young age, I knew. I knew this thing about myself, even if I wasn't ready to admit it. And I think that that's dangerous. I think that that danger has not subsided just because we have more tolerance in this particular place, in this moment in time. Right. I also agree with you. This study's kind of fun, though. Like, I also. Like, I also think it's kind of hilarious, you know, like, one thing that pops into my mind is, like, maybe if you're just surrounded by a lot of men, you. You either choose or don't choose men. Right.

[Selena Simmons-Duffin]

So it sounds like your initial thought is like, okay, this is bunk. But then part of you is like, okay, it's kind of fun, but it's also really dangero. To summarize, it's 45 things at once.

[Justin Torres]

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. So, yeah, I think it's really. I do think it's fun. I think it's worrisome when the science takes itself too seriously.

[Jan Kabatek]

Right.

[Justin Torres]

Because I don't know, 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]

Then again, how could scientists resist that mysterious weird complexity, even if the joy and self expression and community and so many other parts of queerness will always be more than statistics can explain? This episode was produced by Rachel Carlson and edited by our showrunner Rebecca Ramirez. It was fact checked by Britt Hanson. Maggie Luthar was the audio engineer. I'm Selena Simmons. Duffin, thanks for listening to Short Wave from npr. So Justin, I want you to know that I went through this rabbit hole of looking up celebrities like the subset of gay people who are public figures which is very small, and then tried to find the ones who happen to have a bunch of older brothers. Abby Wambach is the youngest of seven and has four older brothers. Ooh, and Nathan Lane is the youngest of four.

[Justin Torres]

I forgot I don't have Nathan Lane. I just think he catch me out

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