New research confirms that having more older brothers increase the likelihood of having a same-sex relationship at some point in mans live. The fraternal birth order effect (FBO) is one of the most well-documented models for determining the biological origin of a person’s sexual orientation. Studies analyzing the FBO suggest that a man is more prone to homosexuality if he has older biological brothers.
The effect of FBO is related to the pregnant woman’s immune response to proteins produced by the male fetus. Proteins enter the mother’s bloodstream and cause the formation of antibodies that affect the sexual development of future children.
These maternal antibodies accumulate during multiple pregnancies in which only male offspring are born, which means that men with more older siblings are more likely to experience homosexuality.
A very recent study using data from the Dutch population registers was carried out to test the theory. These data allow the analysis of more than nine million people born between 1940 and 1990.
Although the data did not include direct indicators of a person’s sexual orientation, they indicated whether the person had ever had a same-sex marriage or registered partnership.
The results show clear evidence of the effects of FBO on homosexuality. In particular, men with one older sibling are 12% more likely to join a same-sex union than men with one older sister.
The order of birth and the total number of siblings are also important. Men who are the youngest in the family are more likely to be homosexual than men who are the oldest, and the gap increases as the total number of siblings increases.
For example, a man is 41% more likely to have same-sex relationships if he has three older brothers, as opposed to three older sisters. However, the number of younger siblings has practically no effect on this factor.
At the same time, women were also analyzed. The researchers found that women were also more likely to have same-sex relationships if they had older siblings. This finding provides tentative support for the argument that maternal antibodies and fetal proteins also interact to affect women’s sexual development.
What can we gather from this?
Everything is quite simple: the number and gender of a person’s biological brothers and sisters play an important role in the development of their sexuality.
This evidence is in line with the view that sexual orientation is an inherited trait and a reflection of one’s true nature, rather than a ‘lifestyle choice‘ or ‘fashion trend‘, as is often believed.
It also refutes claims that homosexuality can be ‘taught’ (for example, by educating about sexual diversity in schools) or ‘passed on’ (for example, by adopting children as same-sex couple).
Understanding the mechanisms of sexual orientation can provide insight into what makes people who they are and helps to normalize the spectrum of sexual diversity across people.