Selena Simmons-Duffin

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.

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.

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

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.

With an exception.

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

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.

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.

Let's start with the basics of how the fraternal birth order effect plays out.

I asked Scott Simonina about this.