When a woman is expecting a male child, there is an immune response that targets the area of the fetus brain which is connected to sexual orientation.
With every male child the woman bears, this immune response is heightened. Scientists analyzed the data from 1000 men which included adopted boys as well as those brought up with stepbrothers and stepsisters.
The study revealed that growing up with stepbrothers did not impact sexual orientation, but having biological brothers did, even if they were raised in different homes.
Antibodies are produced when a mother is pregnant with a boy and this attacks the region in the brain that controls sexuality and this risk is elevated with the birth of each son.
However, researchers found no connection between many sisters in the family and risk of homosexuality.