Selena Simmons-Duffin

He's a psychology professor at Stetson University.

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

Okay.

And so how big is the fraternal birth order effect?

Got one older brother, like me?

Your chance of being gay nudges up to about 2.6%.

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.

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.

And that immune response has some effect on the development of subsequent male fetuses.

However, the plausibility of this hypothesis is newly up for debate.