A study published in 1962 claimed that a man’s likelihood to be gay is proportional to the number of older brothers he has. This phenomenon was called the fraternal birth order effect. After more than 50 years, researchers have finally discovered an explanation for this odd effect. It turns out the event is related to the presence of the Y chromosome.
Gay men are more likely to have an older brother
Women do not possess the Y chromosome, so they cannot produce the protein related to this type of chromosomes. However, when women get pregnant with a baby boy, such proteins enter their blood. The first reaction of the organism is to produce antibodies, since the substances are foreign.
The antibodies do not affect the babies, but they remain preserved in the woman’s body. As soon as they get pregnant with another boy, these antibodies get activated, penetrate the placenta, and end up in the brain of the fetus. They perform a few alterations, so the boy might develop a different sexual attraction as he grows up.
Sexual orientation is complex, and is influenced by a series of factors. However, the fraternal birth order effect is among them, and researchers have finally managed to answer some more answers regarding the phenomenon.
This was the first study to explain the fraternal birth order effect
For the study, researchers took plasma from both men and women. Among the women, there were mothers with daughters, heterosexual sons, gay sons, and women who had no children. They looked for the protein related to the Y chromosome, and discovered that those women who had gay sons who also had older brothers had the highest concentration of the protein of all the participants.
Therefore, the genes in the brain which control sexual orientation appear to be related to this immune response present in mothers with more sons. This study is significant, since it’s the first that explains how this fraternal birth order effect, and the first that offers a possible biological explanation to why some men are gay.