The order your siblings were born in may play a role in identity and sexuality : Short Wave It's National Siblings Day! To mark the occasion, guest host Selena Simmons-Duffin is exploring a detail very personal to her: How the number of older brothers a person has can influence their sexuality. Scientific research on sexuality has a dark history, with long-lasting harmful effects on queer communities. Much of the early research has also been debunked over time. But not this "fraternal birth order effect." The fact that a person's likelihood of being gay increases with each older brother has been found all over the world – from Turkey to North America, Brazil, the Netherlands and beyond. Today, Selena gets into all the details: What this effect is, how it's been studied and what it can (and can't) explain about sexuality.

Interested in reading more about the science surrounding some of our closest relatives? Check out more stories in NPR's series on The Science of Siblings.

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The order your siblings were born in may play a role in identity and sexuality

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EMILY KWONG, BYLINE: You're listening to SHORT WAVE from NPR.

SELENA SIMMONS-DUFFIN, HOST:

Hi, SHORT WAVErs. 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 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 a sudden visibility of underground queer culture.

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TORRES: And then the concern is that there's something pathological happening with these people.

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."

TORRES: My novel is interested in these kind of pre-Kinsey sexology studies, 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.

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

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.

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 Kabatek, 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.

SIMMONS-DUFFIN: With an exception.

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SIMMONS-DUFFIN: The one thing that researchers zeroed in on that seemed to be actually real was this.

KABATEK: Men, specifically, who exhibit attraction to the same sex are likely to have more older brothers than other types of siblings.

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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 U.S., and Turkey, and Canada, and the Netherlands, and Samoa, and Mexico and Brazil - pretty much everywhere it's been studied. Lots of gay men are the youngest brothers in their families.

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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.

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SIMMONS-DUFFIN: Today on the show, the fraternal birth order effect - what it is, how it's been studied 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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SIMMONS-DUFFIN: Let's start with the basics of how the fraternal birth order effect plays out. I asked Scott Semenyna about this. He's a psychology professor at Stetson University.

How likely is it at baseline that someone will be gay?

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

SIMMONS-DUFFIN: OK. And so how big is the fraternal birth order effect?

SEMENYNA: 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.

SIMMONS-DUFFIN: Got one older brother, like me? Your chance of being gay nudges up to about 2.6%.

SEMENYNA: And then that probability would increase another 33% if there was a second older brother.

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.

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SIMMONS-DUFFIN: And the odds can add up and translate to higher chances of younger brothers actually getting married to someone of the same sex. Here's Jan Kabatek, the researcher in Melbourne, talking about his research.

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.

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 Semenyna explains, there's been a leading theory to explain this - the maternal immune hypothesis. *

SEMENYNA: * 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.

SIMMONS-DUFFIN: And that immune response has some effect on the development of subsequent male fetuses. In 2017...

SEMENYNA: 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 no sons whatsoever.

SIMMONS-DUFFIN: However, the plausibility of this hypothesis is newly up for debate. Recently, 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 - with a twist.

KABATEK: Interestingly enough, and this is quite different from what has been done before, we also showed that the same association manifests for women.

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 threw into question the maternal immune hypothesis.

KABATEK: One option is that the hypothesis works for both men and women. Of course, there can be also other explanations. 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.

SIMMONS-DUFFIN: Is there another way to think about this besides statistical oddity? Like, is there something more profound or deeper to think about it?

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.

SIMMONS-DUFFIN: OK. So how does all of this sound to Justin Torres, a queer person who actually fits this pattern?

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

SIMMONS-DUFFIN: OK. So, Justin, in one way I'm like, this is kind of fun. Like, it's concrete.

TORRES: Yeah. Yeah.

SIMMONS-DUFFIN: And it's everywhere, all over the world that this thing happens.

TORRES: Yeah.

SIMMONS-DUFFIN: 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.

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 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 that the study is kind of fun, though. Like...

(LAUGHTER)

TORRES: ...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 either choose or don't choose men, right?

SIMMONS-DUFFIN: So it sounds like your initial thought is like OK, this is bunk. but then part of you is like, OK, it's kind of fun, but it's also really dangerous. To summarize, it's 45 things at once.

TORRES: It's 45 things at once. Yeah. I mean, I love birth order in general. It's 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, right? 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.

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.

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SIMMONS-DUFFIN: This episode was produced by Rachel Carlson and edited by our showrunner Rebecca Ramirez. It was fact-checked by Brit Hanson. Maggie Luthar was the audio engineer. I'm Selena Simmons-Duffin. Thanks for listening to SHORT WAVE from NPR.

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SIMMONS-DUFFIN: So, Justin, I want you to know that I went through this rabbit hole of looking up celebrities...

(LAUGHTER)

SIMMONS-DUFFIN: ...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.

TORRES: Yeah.

SIMMONS-DUFFIN: Abby Wambach is the youngest of seven...

TORRES: Ooh.

SIMMONS-DUFFIN: ...And has four older brothers.

TORRES: Ooh.

SIMMONS-DUFFIN: And Nathan Lane is the youngest of three.

TORRES: Oh, my God, Nathan Lane. I just think he's the cat's meow.

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