Men with older brothers are more likely to be gay, according to a study published Wednesday in the journal Proceedings of the Royal Society B. This new report builds upon previous research regarding male birth order and homosexuality, though there is still no decisive conclusion as to why there’s a link between the two.
In cross-analyzing data from 10 scientific studies with more than 5,000 subjects, researchers found that men with older brothers were 38 percent more likely to identify as gay.
“We certainly thought that the second meta-analysis would confirm the fraternal birth order effect, and it did,” Dr. Ray Blanchard, the study’s lead author and a professor of psychiatry at the University of Toronto, told NBC News. “This is something that has been shown in previous research in various countries, but one of the challenges has been that that research has varied drastically in looking at family samples of various sizes.”
The study, which compared the probability of the youngest brother in a two-son family being gay as compared to the oldest son, did not include female subjects, as previous research has established that women do not influence the sexual orientation of their younger siblings, according to Blanchard.
While this study did not determine a concrete reason for the correlation between a man having an older brother and identifying as gay, Blanchard said that it “points toward the need for more research on the 'maternal immune hypothesis'” — a theory Blanchard introduced in 1996 that suggests that a mother is exposed to “male-specific substances” during pregnancy with or the birth of a son. According to proponents of the theory, the mother’s immune response is changed during the pregnancy of a boy, increasing the likelihood that a subsequent male child would be gay.
“It’s reasonable to speculate that male-specific proteins may cause mothers to develop an immune response that causes them to release antibodies that could affect sexuality" of a second son, Blanchard said. “But not everything that’s biological is genetic or environmental is social, so this research is just the foundation for further analysis.”
Follow NBC Out on Twitter, Facebook & Instagram