Having a greater number of older brothers increases the probability of a person entering a same-sex union at some point in their lives. This is according to a research – tiled “Are Sibship Characteristics Predictive of Same Sex Marriage? An Examination of Fraternal Birth Order and Female Fecundity Effects in Population-level Administrative Data from the Netherlands” by Christine Ablaze, Jan Kabatek and Francisco Perales – that was published in the Journal of Sex Research.
The origin/s of sexual orientation remain contentious. For those who support a biological origin of human sexual orientation, the “fraternal birth order effect” is one of the most well-documented patterns. In this hypothesis, men’s propensity for homosexuality is believed to increase with the number of their older biological brothers. Supposedly, this is because of a mother’s immune reaction to proteins produced by a male fetus, i.e. the proteins enter the mother’s bloodstream and trigger the production of antibodies influencing the sexual development of subsequent children. Since the antibodies accumulate over successive pregnancies with male fetuses, the younger men are then more likely to experience same-sex attraction.
The researchers in the newer study used data from Dutch population registers, allowing them to follow the life trajectories of more than nine million people born from 1940 to 1990. As caveat: the research did not look at sexual orientation of people, only if they entered same-sex unions.
The researchers found a “clear evidence of a fraternal birth order effect on homosexuality.”
Specifically, men with one older brother are said to be 12% more likely to enter a same-sex union than men with one older sister, and 21% more likely than men with just one younger brother or sister.
The researchers also found that men who are the youngest sibling are more likely to enter a same-sex union than men who are the oldest sibling, with the differences growing larger as the total number of siblings increases. For instance, the probability of a man entering a same-sex union is 41% greater if he has three older brothers versus three older sisters, and 80% greater than if he has three younger brothers.
The same pattern of results were found among women – i.e. women are also more likely to enter a same-sex union if they have older brothers.
The results tell a clear story, stated the researchers – i.e. the number and sex of a person’s siblings play an important role in the development of their sexuality.