In the process of looking for something to study in my Independent Study in Advanced Topics in Psychology, I stumbled across an interesting theory called “The Older Brother Effect,” a theory that was developed in response to an observation that many gay males seemed to have multiple older biological brothers. This seemed to be right up my alley.

In taking a closer look, there has been some interesting research into what happens biologically during the pregnancy of male children. Here is my understanding of the science behind it.

During pregnancy, many proteins are produced that help the embryo grow and develop in the womb. It was found that the H – Y antigen, a protein produced during a male pregnancy, passes through the placental barrier to the fetus. This triggers the production of antibodies in the mother’s bloodstream that influence sexual development. 

These antibodies play a role in neutralizing Y – proteins in the NLGN4Y gene that affect sexual differentiation and preference during development. The theory is that the build up of these antibodies, because of multiple male pregnancies, may lead to sexual development that is more typically female in a subsequent male pregnancy, including attraction to males. Essentially, there is a genetic modification caused by the alteration of the NLGN4Y that results in more “feminine” traits. The NLGN4Y gene is found within both females and heightened in homosexual males.

These antibodies can build up over multiple pregnancies and having a biological older brother can increase the chances of being homosexual by around 33%.

So, what does this ultimately mean? Everything in psychology is a mix between both nature and nurture: environment and biology. Having multiple male pregnancies certainly looks like a factor that contributes to the biology of homosexual men. 

https://findanexpert.unimelb.edu.au/news/37927-research-confirms-men-with-older-brothers-are-more-likely-to-be-gay–suggesting-same-sex-attraction-has-a-biological-basis 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10441532/#:~:text=The%20fraternal%2Dbirth%20order%20effect,as%20the%20maternal%20immune%20hypothesis. 

https://pubmed.ncbi.nlm.nih.gov/11534970/ 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4145994/ 

https://en.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation