A man’s sexual orientation is determined before birth, according to research in a study which adds fuel to the gay genetics debate.

Researchers at Brock University in Ontario, Canada discovered that conditions inside the womb have an impact on the baby’s orientation rather than the way they are raised.

The findings came after the author, Professor Anthony Bogaert explored previous research which found being gay was linked to the number of older brothers in the family.

Professor Bogaert writes in the study, which is published this week in the journal Proceedings of the National Academy of Sciences, “These results support a prenatal origin to sexual orientation development in men and indicate that the fraternal birth-order effect is probably the result of a maternal `memory’ for male gestations or births.”

He studied over 900 heterosexual and homosexual men in Canada who had either biological or non-biological brother and examined the impact of how they were related and the amount of time spent together.

The results showed that the number of biological siblings had a bigger effect on homosexuality, therefore suggesting it is down to nature and not nurture.

“The number of biological older brothers, including those not reared with the participant . . . increases the probability of homosexuality in men,” the study reads.