Gay men have a greater number of older brothers as compared to heterosexual men, which indicates that having a higher number of older male siblings plays a part in affecting a man’s sexual orientation. It is a well-known finding in sexual science and is termed as fraternal birth order effect.

A new study has now pointed that growing levels of antibodies in the immune system of the mother could play a part in determining a man’s sexual preference. A team of researchers from various universities in Canada. published the study in the journal PNAS on Dec. 11.

Maternal Antibodies Play A Part

Brock University’s Anthony Bogaert thinks that some females who are gestated with males produce antibodies that target a Y chromosome-created protein. The immune system generates antibodies to detect foreign molecules created from harmful bacteria. Pregnant women, however, also generate antibodies against fetal molecules, in cases like when the fetus has another blood group.

Bogaert’s team pondered if maternal antibodies might play a part in impacting sexual orientation. The researchers collected blood samples from 142 pregnant women and tested them for antibodies to a specific brain protein, known as NLGN4Y, which is only produced in males.

The team thought NLGN4Y would be an ideal candidate due to its significant part in how neurons communicate with one another, making it comparatively simple for antibodies to detect it.

The scientists found that women who had homosexual sons with older male siblings had the most increased levels of antibodies against NLGN4Y. The next in line were mothers of homosexual sons with no older male siblings. Mothers who had heterosexual sons had lower levels of these antibodies, whereas sonless women had the lowest level.

Brain Wiring

The research team suggested that with every male baby some women have, these antibodies build up in their bodies. At increased concentrations, there is a chance that their impact on the protein they target results in an alteration in the development of the brain, which can influence sexual orientation.

“I wouldn’t say we’ve solved the fraternal birth order effect puzzle, but we are getting close to finding a mechanism,” Bogaert said.

The editor of the study paper, Marc Breedlove, from the Michigan State University, said the research was important because it offered a reasonable mechanism for explaining the fraternal birth order effect. Given that NLGN4Y is known to be crucial in the formation of the synapse, one can see how antibodies in mothers might impact the fetal brain’s wiring, which might explain why younger sons are most likely to grow up as homosexual.