Chapter 1: What is the main topic discussed in this episode?
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You're listening to Shortwave from NPR. 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 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.
There's a sudden visibility of underground queer culture. And then the concern is that there's something pathological happening with these people.
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.
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.
The queer people scientists were studying were also living in a world where this facet of their identity was dangerous.
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.
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Chapter 2: How does the number of older brothers influence sexuality?
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.
Most of it fell flat, meaning that we still have very little idea about what underlies the origins of sexual orientation.
With an exception. The one thing that researchers zeroed in on that seemed to be actually real was this.
Men, specifically, who exhibit attraction to the same sex, are likely to have more older brothers than other types of siblings.
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 Brazil. Pretty much everywhere it's been studied. Lots of gay men are the youngest brothers in their families.
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.
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 Shortwave, the science podcast from NPR.
Support for NPR and the following message come from the William and Flora Hewlett Foundation, investing in creative thinkers and problem solvers who help people, communities, and the planet flourish. More information is available at hewlett.org.
Let's start with the basics of how the fraternal birth order effect plays out. I asked Scott Simonina about this. He's a psychology professor at Stetson University. How likely is it at baseline that someone will be gay?
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Chapter 3: What is the history of scientific research on sexuality?
And then that probability would increase another 33% if there was a second older brother.
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's Jan Kabatek, the researcher in Melbourne, talking about his research.
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.
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 Simonina explains there's been a leading theory to explain this, the maternal immune hypothesis.
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.
And that immune response has some effect on the development of subsequent male fetuses.
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 no sons whatsoever.
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, with a twist.
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