In the 1990s, Blanchard and Bogaert proposed the Fraternal Birth Order Effect (FBOE). Men with more older brothers were more likely to be gay. "The odds of having a gay son increase from approximately 2% for the first born son, to 3% for the second, 5% for the third and so on".

https://www.astralcodexten.com/p/how-are-the-gay-younger-brothers