Studies find a link between having older brothers and being gay. Does it matter? : Shots - Health News Studies worldwide show that queer people tend to have more older brothers than other kinds of siblings. Justin Torres, a queer novelist and the youngest of three brothers, asks: Should it matter?

Gay people often have older brothers. Why? And does it matter?

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AILSA CHANG, HOST:

Did you know that people who are gay are more likely to have older brothers than other kinds of siblings? There's actually research going back decades into this apparent connection between family makeup and sexuality. For our series on the science of siblings, NPR's Selena Simmons-Duffin explores this phenomenon.

SELENA SIMMONS-DUFFIN, BYLINE: Justin Torres is a novelist. He's also queer, and he's the youngest in his family.

JUSTIN TORRES: I grew up with two older brothers. It was just the three of us, and it was pretty wild.

SIMMONS-DUFFIN: He grew up in New York State and wrote about the three-brother dynamic in his first novel, "We The Animals."

TORRES: As somebody who's the youngest of three, I always have these intense moments of bonding with other people who are the youngest of three or four or five, right? Like it's - it is a particular lived experience, I think.

SIMMONS-DUFFIN: In his new novel, "Blackouts," which won the National Book Award last year, Torres grapples with how scientists have studied sexuality.

TORRES: My novel is kind of 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: But despite their efforts, the researchers didn't find much.

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: That's Jan Kabatek, a modern-day researcher at the University of Melbourne. There is one exception, though. The thing that scientists zeroed in on that seemed to actually hold up was this.

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

SIMMONS-DUFFIN: In other words, more families looked like that of Justin Torres, where the youngest brother is queer. In the 1990s, this was dubbed the fraternal birth order effect. In the years since, it's been found again and again. Scott Semenyna is a psychology professor at Stetson University.

SCOTT SEMENYNA: This effect seems to show up pretty much everywhere that it's examined, from different locales scattered around Canada and the United States, but it goes well beyond that. There's been now many confirmations that this pattern exists in countries like Samoa. It exists in southern Mexico. It exists in places like Turkey and Brazil.

SIMMONS-DUFFIN: Here's how the fraternal birth order effect plays out. At baseline, Semenyna says, the chance someone will be gay is really pretty small, like 2%.

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 3.5%. If you have five older brothers, your chance of being gay is about 8%, so four times the baseline probability, but still kind of small.

SEMENYNA: The vast majority of people who have a lot of older brothers are still going to come out opposite-sex attracted.

SIMMONS-DUFFIN: Also, plenty of gay people have no brothers at all, or they're the oldest in their families. This is definitely not the only influence on a person's sexuality. Still, the consistency of this effect makes you wonder why. Why would it be that the more older brothers someone has, the more likely they are to be gay? Semenyna says 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. However, there has been a recent development when it comes to this explanation. Jan Kabatek, the researcher in Melbourne, analyzed a huge sample - more than 9 million people. He and his colleagues found yet again 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: Women who were in same-sex marriages were also more likely to have older brothers than other types of siblings. He says now the game is on for scientists to figure out why.

KABATEK: It's for the prospective research to make this clearer.

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

TORRES: It is the undercurrents that worry me.

SIMMONS-DUFFIN: Talking about this brings to mind a memory from watching daytime TV as a kid.

TORRES: You know, this is like the late '80s, early '90s. 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?

SIMMONS-DUFFIN: Even if tolerance for queer people in American society has grown a lot since then, he says...

TORRES: I think that tolerance waxes and wanes, and I worry about the line of thinking.

SIMMONS-DUFFIN: At the same time, the idea that there's a connection with gay people being the youngest kids in their families, he also finds it kind of hilarious.

TORRES: 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: In other words, it's fun to think about, but maybe not deeper than that.

TORRES: 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?

Selena Simmons-Duffin, NPR News.

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