Scott Simonina

Somewhere around 2% to 3%.

We can call it 2% just for the sake of simplicity.

The fraternal birth order effect shows about a 33% increase in the probability of male same-sex attraction for every older brother that you have.

And then that probability would increase another 33% if there was a second older brother.

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.

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.