The fraternal birth effect shows people are born gay

Question: Are people born gay?
Position1 of 4›: Yes, people are born gay
Argument‹3 of 6›: The fraternal birth effect shows people are born gay

The more older brothers you have, the higher the likelihood of you being attracted to members of the same sex.

The argument

The fraternal birth effect is offered as evidence that sexual orientation is settled before birth rather than shaped by upbringing. The observation itself is a statistical regularity: for every older brother someone has, they have a 33% greater chance of being gay. The effect is cumulative rather than a threshold, since each additional older brother raises the likelihood again, which is what makes it a linear correlation rather than an isolated association. The decisive feature of the finding is what it excludes. The effect does not apply to adopted siblings. If growing up with older brothers were what mattered, through the household dynamic, the role models available, or the way a younger child is treated, then an adopted older brother would count the same as a biological one, because the social environment is identical in both cases. It does not count the same. What distinguishes the two situations is not the home a child is raised in but the pregnancies their mother has previously carried. That isolates the mechanism to something occurring before the child is born. A correlation that tracks the sequence of a mother's biological births, and disappears when the sibling arrives by any other route, points to genetic or hormonal impacts operating during gestation rather than to anything in the child's later experience. The child has no awareness of their birth order at the point the relevant process would have to occur. Because the pattern is biological in origin and prenatal in timing, proponents conclude that the fraternal birth effect shows people are born gay rather than becoming so.

Context

Studies indicate that the more older brothers you have, the more likely you are to be born gay.

Premises

[P1]For every older biological brother a person has, their chance of being gay rises by about 33%, an effect that does not hold for adopted siblings. [P2] Because the correlation tracks biological rather than adopted brothers, it points to genetic or hormonal influences on sexual orientation rather than environment. [C] Therefore, because the fraternal birth effect reflects prenatal biology, people are born gay.

Counter-arguments

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.

Rejecting the premises

[Rejecting P4] If this was the case then as family sizes decrease, so would the number of gay people.