Having older brothers may increase the likelihood of being gay

By Dr. Ananya Mandal, MDDec 11 2017

Scientists have found a distinct pattern that having older brothers raise the chances of the younger sibling being gay. This effect has been termed the “fraternal birth order effect”. They explain the biological reason behind this propensity in their new study that was published this week in the journal PNAS.

The researchers explain in their work that this fact has been known from previous studies. This latest study sheds light on the “whys and wherefores” of this phenomenon. They explain that there are several factors that determine the sexuality of an individual. They note that there is a protein that is associated with the Y chromosome which is present exclusively in men. Women have two X chromosomes while men have an X and a Y chromosome.

This Y chromosome associated protein called NLGN4Y that is important for male brain development and development of attraction towards a partner. As a woman gets pregnant with a boy for the first time, this Y chromosome associated protein is produced in her and leaks into her bloodstream. This protein is unknown to the immune system of the mother and so antibodies are created within her to fight this foreign protein. When she gets pregnant with another boy, these antibodies against the protein cross her placenta and enter the second baby boy’s brain. This changes the male development of the fetus and alters his sexuality. Study author Anthony Bogaert, psychologist and professor in the departments of psychology and community health sciences at Brock University, explained that the development of “sense of attraction” changes in the male fetus as it grows into a baby and then into a man.

Bogaert explained that this protein NLGN4Y is a regulator of how brain cells connect with each other. It could also affect the attraction that develops with time. The immune system of the mother that is altered with the birth of the first boy may change this delicate balance he explained.

There have been studies that show that more number of elder brothers a boy has, the chances of the boy being attracted to men would be greater. They had also noted that raising the younger son away from elder brothers did not reduce the risk of him being gay and adopted younger brothers did not have this raised risk.

For this study Bogaert and his colleagues looked at 142 women and 12 men between ages 18 and 80 years and checked for the protein NLGN4Y and the antibodies against it. They noted that the highest levels of NLGN4Y was seen in women with gay younger sons compared to women who did not have any sons or women who had only heterosexual sons.

“I wouldn’t say we’ve solved the fraternal birth order effect puzzle, but we are getting close to finding a mechanism,” says Bogaert. They agree that this was a small study and further larger studies are necessary to draw stronger and more accurate conclusions.

Reference: http://www.pnas.org/content/early/2017/12/05/1705895114