Gay genes? Upbringing? Parents? Peer influence? When it comes to predicting if a male child will grow up gay, none of those factors compare to knowing how many older brothers he has. 

This observation is called the Fraternal Birth Order Effect (FBOE). A crazy amount of research has shown that the biggest predictor of a male child being gay is how many older brothers went through his mother's womb! 

Wait, what?

Yep. It doesn't matter how many older sisters you have. Or how many adopted older brothers? It doesn't matter if those older brothers are raised with you or moved away at birth. All that matters is how many of those older brothers were birthed by your biological mother.

Why?

This is still under investigation, but it looks like mothers increasingly start making antibodies to their son's NLGN4Y protein. Either this or another important male protein gets scavenged by mom's immune system, leaving less of this signal to get to the developing male fetus. As this signal goes down, critical milestones are subtly altered, resulting in male children being more and more likely to be gay.

This can't possibly be a big effect, can it?

In fact, it accounts for about 14% of all gay men, just by itself! In large families, this effect gets huge. The last boy of one gay son among six sons is almost 50%! Check out here for our free simulator where you can play around with data! 

Keep in mind that in most cases, this effect only partially determines sexual orientation. And even when it is a huge determination, there are exceptions. For example, this has no effect at all on left-handed sons, but no one knows why.

Bottom line: sexual orientation is surprising!