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Men with older brothers more likely to become gay, study suggests

A paper published in the journal PNAS has suggested a biological explanation

Men with older brothers have an increased chance of being gay – and scientists now think they know why.
Researchers had previously observed that men with older brothers are more likely to be gay, but a paper published in the journal PNAS has suggested a biological explanation.
Previous research showed that for each older brother a man has, the chance he will be gay rises by roughly a third.
The researchers suggest that when a woman becomes pregnant with her first boy, a protein linked to the Y chromosome (which women don’t have) enters the bloodstream.
The mother’s body responds to this ‘foreign’ substance by creating antibodies – and when she is pregnant with a second boy, the antibodies can sometimes enter the brain of the foetus.
Lead author Anthony Bogaert of Brock University says, ‘That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction.’