Is there something about birth order that influences sexuality? According to a fascinating line of scientific research, the answer might be yes—and it’s called the Fraternal Birth Order effect. This theory suggests that the more older biological brothers a man has, the more likely he is to be gay. It’s not about upbringing, parenting, or personality—it’s biological. The effect doesn’t apply to women, or to sisters. It’s specific to men born after older brothers from the same mother. Researchers believe the mother’s body may develop a progressive immune response during male pregnancies. In short, with each son she carries, her immune system may become slightly more sensitised to male-specific proteins. By the time her third or fourth boy comes along, these subtle changes might influence how that boy’s brain develops in utero—particularly in areas connected to sexual orientation. It doesn’t mean every man with older brothers is gay. It just means that, statistically, a man’s chances of being gay rise slightly with each older brother he has. It’s one of the most consistently replicated findings in sexuality research—yet still rarely talked about outside academic circles.
