Past research has suggested that men with at least one older brother are more likely to be gay—a phenomenon called the fraternal birth order effect. Now, a new study by Canadian and German scientists adds more evidence to that theory, and it points to possible antibodies produced by the siblings' mom. NBC News cites research published in the Proceedings of the Royal Society B journal that analyzed data from 10 unrelated studies involving nearly 5,400 male subjects, both homosexual and heterosexual. The scientists found that men who had one older brother had a 38% higher chance of being gay than those without an older bro. Having three older brothers doubled a man's chances of being gay, per Phys.org. One thing the study didn't find: any correlation between birth order and sexuality in women.
Although a reason for this link wasn't found, the researchers have been looking at one theory in particular: the "maternal immune hypothesis," which lead study author Ray Blanchard first put forth more than 20 years ago. Per this theory, cells or cell fragments from male fetuses enter the mother's circulation either during the pregnancy or during childbirth. As a result, the mother produces antibodies against these "foreign," male-specific objects, and those antibodies remain in her body. During her next pregnancy with a male child, those antibodies somehow cross over into the fetus's brain, "bind to male-specific molecules on the surface of neurons and prevent these neurons from 'wiring-up' in a fully male-typical pattern." As a result, the study notes, "the individual will later be attracted to men rather than to women." Still, Blanchard tells NBC, "this research is just the foundation for further analysis." (Read more discoveries stories.)