Studies indicate that the more older brothers you have, the more likely you are to be born gay.
For every older brother someone has, they have a 33% greater chance of being gay. However, this does not apply to adopted siblings. The linear correlation between fraternal births and homosexuality indicates that there are genetic or hormonal impacts that dictate a child’s sexual orientation.
If this argument were true, then as family sizes decrease, the number of gay people should also drop. It would also suggest that in places like China, which adopted a one-child policy, the number of homosexuals would drastically decrease. This has not happened. The number of gay people in society has remained stable despite family sizes decreasing significantly.
[P1] There is a direct link between the number of older brothers you have and the likelihood of being gay. [P2] This link is not visible in adopted siblings. [P3] Adopted siblings have the same upbringing but different genetics. [P4] Therefore, the causes of same-sex attraction is genetic. [P5] Therefore, people are born gay.
Rejecting the premises
[Rejecting P4] If this was the case then as family sizes decrease, so would the number of gay people.