A recent study published by the Proceedings of the National Academy of Sciences (PNAS) sought to uncover the mechanism behind the observation (scientifically known as the fraternal birth order effect) that the more older brothers a male has, the greater the likelihood he is to be gay. Meta-studies have proven the effect across a broad range of studies. Efforts to quantity the effect estimate that for each older brother a man has, the chance that he will be gay increases about 30%.
It turns out that the mechanism is probably a Y-linked protein (called NLGN4Y) linked to brain development. Women do not have a Y-chromosome and thus initially are unlikely to have large amounts of this protein in their system. Indeed being a Y-linked protein, a mother’s body sees the protein as something it is supposed to make anti-bodies against. Thus, when a woman becomes pregnant with a son, the fetus, being male, begins producing the protein NLGN4Y to stimulate its development. At some point the mother becomes “infected” with the protein and begins to produce antibodies to the protein, antibodies that then persist in the mother. The researchers compare it to the same mechanism of a vaccine. The more sons a woman has, the higher the concentration of the antibodies in her body becomes. These antibodies may somehow (in concert with other factors) result in a subsequent male child being gay. The study noted
Our results begin to explain one of the most reliable correlates of sexual orientation in men and provide evidence of a specific biological mechanism underlying men’s sexual orientation: a maternal immune response to a Y-linked protein important in male fetal brain development. A maternal immune mechanism does not exclude other factors (e.g., prenatal hormones, genetics) advanced to explain sexual orientation. Indeed, although most of our key effects were of a notable statistical magnitude (i.e., medium in effect size), it is also clear that only a portion of variation in men’s sexual orientation is accounted for by these effects. Sexual orientation is clearly a complex phenomenon with likely many factors influencing it. (emphasis added)
The study’s author Anthony Bogaert stresses that suggesting homosexuality is a disease or that it is treatable “would be a misuse of this work.” In an interview with Newsweek’s Kate Sheridan (BORN THIS WAY: WHY HAVING AN OLDER BROTHER MAKES MEN MORE LIKELY TO BE GAY), he also tried to emphasize the benefit the finding could have to the LGBT equality movement.
...[T]he findings suggest “that sexual orientation is not easily changed or is not a choice,” he said. A Pew Research Center survey found that as of 2015, less than half of Americans believed that sexual orientation is something set from birth. If that number increases based on research like this, Bogaert said, that could mean society might become more tolerant and accepting in the future.