Men who have older brothers are more likely to be gay than those who do not, according to a recent study.
The Canadian meta-analysis of data from ten previous studies found that men were 38% more likely to identify their sexual orientation as gay if they had older brothers.
Previous research has found that having older sisters is not related to sexuality, and having younger siblings similarly does not seem to have an effect.
The increased likelihood of the second son in a family being gay is known as the ‘fraternal birth order effect’.
The reason behind it remains unknown, but researchers believe the effect may be related to a male-specific immune response during some pregnancies.
Incorporating data from over 5,000 men, the new study built on similar previous research.
The researchers said the results reflected their expectations.
“We certainly thought that the second meta-analysis would confirm the fraternal birth order effect, and it did,” lead author Dr Ray Blanchard of 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.”
Dr Blanchard said that more research was needed on the maternal immune hypothesis, and that the reasons people grow up gay are likely complex.
“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,” he said.
“But not everything that’s biological is genetic or environmental is social, so this research is just the foundation for further analysis.”
Researchers have long been interested in the causes of sexual orientation, with the search for a genetic basis going back decades.
Breakthrough research published last year, based on a study of almost half a million people, reported several genes associated with sexual behaviour but no ‘gay gene’.