Scientists may have discovered why boys with older brothers are more likely to be gay

'Fraternal birth order effect' estimated to impact around 15 per cent of homosexual men

Numerous studies find correlation between birth order of boys and male sexual orientation (Getty Images)

Scientists may have discovered why the more older brothers a boy has, the likelier he is to be gay as an adult.

Known as the “fraternal birth order effect”, numerous studies have found a correlation between the birth order of boys and male sexual orientation.

Now researchers say increasing levels of antibodies in a mother’s immune system could play a role.

Antibodies are proteins produced by the immune system to fight harmful bacteria and viruses. But pregnant women sometimes produce antibodies against fetal molecules, such as when their foetus has a different blood group.

Anthony Bogaert, a professor in social psychology at Brock University, Canada, believes that some women who are pregnant with boys produce antibodies that neutralise a protein made by the Y chromosome; the DNA molecule that determines sex in humans.

Mr Bogaert and his team speculated that these maternal antibodies might play a role in determining the sexual orientation in their sons.

The team took blood from 142 women and the samples were screened for antibodies to a brain protein only found in males, since only boys have the particular protein-producing Y chromosome.

The protein, known as NLGN4Y, plays a role in how neurons communicate with each other and is made at the surface of brain cells, so easier for antibodies to locate.

The researchers found that the mothers of gay sons with older brothers had the highest levels of this antibody, followed by the mothers of gay sons with no older brothers. Mothers with straight sons had even less, and mothers with no sons had the least.

The team speculates these neuron-attacking antibodies build up in women’s bodies every time they are pregnant with a son. The higher the concentration, the likelier the boy is to be gay, the researchers suggest.

“It could affect brain structures that moderate attraction,” Mr Bogaert told the New Scientist. “The mother’s immune response may alter the typical function of these brain structures.”

He added: “I wouldn’t say we’ve solved the fraternal birth order effect puzzle, but we are getting close to finding a mechanism.”

However, at 142 women, the research involved only a small sample size, so no strong conclusions can yet be drawn over the reasons for the fraternal birth effect.

The phenomenon is also only estimated to impact 15 per cent of gay men.