The Science of Siblings is a new series exploring how our brothers and sisters can influence us, from our money and our mental health to our very molecules. Well-being share these stories over the next few weeks.
This is something I learned years ago from discussions in gay bars: gays are often the youngest children in their families. I immediately liked the idea – as the youngest gay sibling, it made me feel like there was a statistical order to things and I fit right in with that order.
When I started reporting on the science behind this, I learned that it was true: there is a well-documented correlation between having older siblings (older brothers in particular) and a person’s chances of being gay. But parts of the story also struck me as strange and dark. I thought of We the animals, Justin Torres’ haunting semi-autobiographical novel about three brothers – the youngest of whom is gay – who grew up in upstate New York. So I called Torres to get his thoughts on the idea.
Torres’ first reaction was to find this considerably less appealing than I did. This makes sense – his latest novel, Power outages = blackout, won a National Book Award last year and tackles the grim history of how scientists have studied sexuality. “My novel focuses on pre-Kinsey sexology studies, particularly this one entitled Sex variants“, he told me. “It’s really influenced by eugenics. They looked for the cause of homosexuality in the body to treat it, cure it or get rid of it. »
Which is why, when he saw my investigation into a statistical finding linking sexuality and birth order, he was suspicious. “To be frank, I find these kinds of studies that look for something rooted in the body to explain sexuality to be kind of stupid. I think they rely on a really binary understanding of sexuality itself “, did he declare.
“That’s fair,” I conceded. But this connection between homosexuality and older brothers has been discovered so many times and in so many places that one researcher told me it’s “a kind of truth” in the science of sexuality.
Rooted in a dark past
The first research on this subject began in the 1940s and 1950s, at the time of research into the causes of homosexuality, in order to be able to remedy it. At the time, the queer people the scientists studied lived in a world where that side of their identity was dangerous. What’s more, the studies themselves didn’t find much, says Jan Kabátek, lead researcher at the University of Melbourne.
“Most of it fell flat,” he told me. “But there is one exception to this, and that is the finding that men, in particular, who show same-sex attraction are likely to have more older brothers than other types of siblings. “
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In the 1990s, this was dubbed the “birth order sibling effect.” In the years that followed, it was found again and again, all over the world.
“This trend has been documented in Canada and the United States, but it goes well beyond that,” says Scott Semenyna, a psychology professor at Stetson University. “There is now ample confirmation that this pattern exists in countries like Samoa. It exists in southern Mexico. It exists in places like Turkey and Brazil.”
Large study, consistent results
An impressive recent study found that this trend held up when analyzing a large sample – more than 9 million people in the Netherlands. This confirmed all these previous studies and added an extra twist.
“Interestingly – and this is quite different from what has been done before – we also showed that the same association occurs for women,” says Kabátek, one of the study’s authors. Women in same-sex marriages were also more likely to have older brothers than other sibling types.
To begin with, the chances of a person being gay are pretty low. “Somewhere between 2 and 3 percent — we can call it 2 percent just for simplicity’s sake,” says Semenyna. “The fraternal birth order effect shows that you will experience about a 33% increase in the likelihood of male same-sex attraction for each older brother you have.”
The effect is cumulative: the more older brothers a person has, the taller they are. If you have an older brother, your probability of being gay rises to about 2.6%. “And then this probability would increase by another 33% if there was a second older brother, to around 3.5%,” says Semenyna.
If you have five older brothers, your chances of being gay are about 8%, or four times the baseline probability.
The author, Selena Simmons-Duffin, at age 3, with her brother, David Simmons-Duffin, at age 5.
The Simmons-Duffin family
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The Simmons-Duffin family
The author, Selena Simmons-Duffin, at age 3, with her brother, David Simmons-Duffin, at age 5.
The Simmons-Duffin family
However, even 8% is quite low. “The vast majority of people who have many older brothers will always be attracted to the opposite sex,” says Semenyna. Additionally, many gay men have no brothers or are the oldest in their family. Having older brothers is certainly not the only influence on a person’s sexuality.
“But the simple fact that we observe such strong effects, relatively speaking, implies that there is a good chance that there is, at least partially, a biological mechanism driving these associations,” says Kabátek.
A hypothesis, but no definitive mechanism
For decades, the leading candidate for this biological mechanism has been the “maternal immune hypothesis,” says Semenyna. “The basic version of this hypothesis is that when a male fetus develops, the male’s Y chromosome produces proteins that will be recognized as foreign by the mother’s immune system and somehow forms an immune response to these proteins. ”
This immune response has some effect on the development of subsequent male fetuses, explains Semenyna. The plausibility of this hypothesis was reinforced by a 2017 study that found “that mothers of homosexual sons have more of these antibodies targeting these male-specific proteins than mothers of sons who are not homosexual or mothers who are not homosexual.” ‘have no sons,’ he said. .
But now that Kabátek’s study of the Dutch population found that this pattern was also present among women in same-sex marriages, new questions arise about whether this hypothesis is correct.
“One option is that the immune hypothesis works equally well for men and women,” says Kabátek. “Of course, there may be other explanations. It is up to prospective research to clarify this.”
Fun to think about, but worrying too
In a way, I tell Justin Torres, this effect seems simple and fun. This is a concrete statistical finding, documented around the world, and there is an intriguing hypothesis as to why this may be occurring biologically. But darker undercurrents in all of this worry me, like bringing up a dangerous idea that becoming gay in the womb is the only version of homosexuality that’s real — or a repackaged version of the old idea that mothers are to be “blamed”.
“It’s the underlying currents that worry me a lot,” he replies. “I remember when I was a kid – I have this memory of watching daytime TV. I had to stay home from being sick at school in the late 80s or early 90s. The host asked the audience and said: ‘If there was a (during pregnancy) and you could find out if your child was gay, would you have an abortion? I remember being so horrified and disturbed seeing all these? hands go up in the audience – I felt so hated at that young age, I knew this thing about myself, even if I wasn’t ready to admit it.
Although tolerance for queer people in American society has grown a lot since then, he says, “I think tolerance waxes and wanes, and that way of thinking worries me.”
At the same time, he agrees that the idea of a bond with gay people being the youngest children in their families is pretty hilarious. “One thing that comes to mind is that maybe if you’re surrounded by a lot of men, you either choose or you don’t choose men, right?” he’s laughing.
Essentially, in his opinion, it’s fun to think about, but probably no deeper than that.
“As a humanist, I just don’t know why we have to look for explanations for something as complex and joyful and strange as sexuality,” Torres says.
Again, it is unlikely that scientists will be able to resist this mysterious and strange complexity. Even though joy, self-expression, community, and so many other aspects of homosexuality and sexuality will always be more than statistics can explain.
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