Scientists: Having Older Brothers Raises Odds of Being Gay

Canadian scientists have found that the more older male siblings a man has, the greater the chance he will be gay. In a series of studies reported in the UK Daily Mail Online, researchers discovered that each older brother raises a man's odds of being a homosexual by a third.

"It's likely to be a prenatal effect" said Anthony F. Bogeart back when his original findings were released in 2006. "This and other studies suggest that there is probably a biological basis for homosexuality."

The phenomenon is nature, not nurture, say docs from Brock University in Ontario. Rather than being attributed to any differences in the way a youngest son is raised, it has a biological basis.

Scientists say that a woman mounts an immune response in her womb while pregnant with a son. This increases with each male baby, "raising the odds of 'feminising' the foetus' developing brain.'"

A study of 1,000 men, including some who were adopted or brought up with stepsiblings, revealed that having many brothers raised a man's odds of being gay. This correlation only occurred among blood brothers, however. Stepbrothers did not have an effect, but biological brothers who were brought up separately did. No similar link has been found between having many older sisters and becoming a lesbian.

Each older brother raised by one-third the odds that a man will be gay. If a firstborn son has a 3 percent chance of being gay, the figure for the second son will be 4 percent. By the fourth son, the odds of the so-called "fraternal birth order effect" will have doubled.

Canadian scientists now believe that carrying a male baby triggers an immune response in the mother, creating antibodies that attack part of the foetus' brain linked to sexual orientation. Genetics and exposure to hormones are also thought to be important.

The idea that prenatal mechanism may influence sexual orientation has been around for a couple of decades. In 1996, Bogaert and Ray Blanchard first correlated sexual orientation in men with older brothers. In this latest study, Bogaert pitted prenatal against postnatal by examining a fourth group of subjects who had been adopted or raised with half- or stepsiblings.

According to an article in Scientific American, he believed that just having older brothers was the correlation. But he discovered that only biological older brothers predicted sexual orientation in men. He also discovered that upbringing was not an impact, but that having an older brother -- even if a young man did not grow up in the same household as him -- increased his odds of being gay.

"So what that means is that the environment a person is raised in really makes not much difference," said Bogaert.