Having a son triggers a physical reaction in a mother – making it more likely later boys will grow up to be gay, a study suggests.

The research finding has been triggered by one of the unexplained quirks of biology: the more older brothers a man has, the more likely he is to be gay, the Daily Mail reports.

Now scientists claim having a male child triggers an immune reaction in the mother that can alter the brain of the growing baby.

The authors are careful to stress that several factors are involved in explaining sexuality – but say that the immune response helps to explain something that has long puzzled scientists.

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The authors of the report, led by Toronto University, published their study in Proceedings of the National Academy of Sciences.

"Our study is a major advance in understanding the origins of sexual orientation in men by providing support for a theorized but previously unexamined biological mechanism—a maternal immune response to a protein important in male foetal brain development," they say.

They add the research begins "to explain one of the most reliable correlates of male homosexuality: older brothers."

The chance of any male son being gay is estimated to be around 3 per cent. But for someone who has three older brothers, the chance doubles to 6 per cent.

While girls have two X chromosomes – the genes that dictate a child's biological sex, boys have an X and a Y chromosome.

And it is the 'Y' chromosome which creates the biological differences that make a child male that triggers an immune reaction in a mother.

In response to having a baby boy, her body reacts against the Y chromosome, and creates an antibody called anti-NLGN4Y.

Antibodies are a reaction to "foreign" bodies invading the body – in this case a male embryo.

After having a son, a woman builds up a large supply of the antibody, and this can affect the brain development in later male children.

The authors are careful to point out the link between the antibody and the brain structure is an association, but not proof.

The authors write that they suspect "some mothers develop antibodies against a Y-linked protein important in male brain development".

They add: "This effect becomes increasingly likely with each male gestation, altering brain structures underlying sexual orientation in their later-born sons."

The authors write: "After statistically controlling for number of pregnancies, mothers of gay sons, particularly those with older brothers, had significantly higher anti-NLGN4Y levels than did the control samples of women, including mothers of heterosexual sons."

The authors say that their findings "suggest an association between a maternal immune response to NLGN4Y and subsequent sexual orientation in male offspring."