But it has not been clear if this is a prenatal effect or a psychosocial effect, related to growing up with older male siblings.
To investigate, Bogaert studied 944 gay and straight men, including several who were raised with adopted, half- or step-siblings. He reasoned that if the relationship between having older male siblings and homosexuality was due to family environment or child-rearing practices, it would be seen whether or not a man's older brothers were biological or adopted, according to Reurters.
According to the theory, a mother's immune system interprets and remembers male - but not female - fetuses as foreign and creates "maternal anti-male antibodies" that affect the part of the brain that determines sexuality, Channel News Asia reports.
The effect is cumulative, thus is more likely to cause homosexuality in the last of several sons born to the same mother.
Female foetuses are not affected because mothers are themselves female and no antibodies are created during gestation of daughters.