Jan Kabatek

Most of it fell flat, meaning that we still have very little idea about what underlies the origins of sexual orientation.

Men, specifically, who exhibit attraction to the same sex, are likely to have more older brothers than other types of siblings.

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.

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.

Interestingly enough, and this is quite different from what has been done before, we also showed that the same association manifests for women.

One option is that the hypothesis works for both men and women.

Of course, there can be also other explanations, and we still have not really good idea what kind of explanations these could be, but it's for the prospective research to make this clearer.

I don't think of this as just a statistical oddity.

It actually fits quite well into a broader realm of research, which deals with sibling influences.

But just the fact that we are observing effects that are so strong, relatively speaking,

implies that there's a good chance that there is at least partially some biological mechanism that is driving these associations.

Whether this mechanism is the maternal immune hypothesis is an open question, but I wouldn't be completely dismissive of that possibility.