New research says that boys have a greater chance of becoming gay if they have older brothers.
Anthony Bogaert, of Brock University, Ontario, Canada, says a man's sexual orientation may be determined before he is born by the maternal response to carrying male foetuses.
Bogaert says the research provides the strongest evidence yet of a biological basis for male homosexuality and supports the theory of a pre-natal origin to sexual orientation development in men.
A decade or so ago researchers discovered that the more older brothers a boy had, the greater chance he had of being homosexual.
This was put down at the time to the psychological effect of having older brothers on the family dynamics, which possibly affected sexual orientation.
According to the Canadian researchers for each brother that precedes him, a boy's likelihood of growing up gay increases by a third.
But this apparently only applies to brothers with the same biological mother and is not the case where there are older adopted or stepbrothers with a different mother.
But the increased chance of homosexuality applied even where men had older full brothers who had been raised separately in a different home, offering further evidence for a biological effect.
The findings suggest that in a proportion of gay men, their orientation is heavily influenced by biological factors they experience before they are born, and not by the way they are brought up or choices they make later in life.
The findings clearly indicate that conditions in the womb before birth, and not the subsequent family environment, are responsible for whether a boy becomes gay.
The theory that having a large number of older brothers might influence male sexuality is not new and was first suggested in 1997.
A study led by Ray Blanchard, of the Centre for Addiction and Mental Health, Toronto, found that gay men were more likely to have lots of elder brothers than both straight men and lesbians.
Previous research has also suggested that genetics and the womb environment can have a major impact on sexual preferences in both men and women.
Why having older biological brothers affects male sexuality is unclear, but many researchers theorise that it is a reflection of the way a mother’s immune system reacts to carrying lots of male foetuses.
Males have a Y chromosome and females do not, and it is thought that a mother’s body may be more likely to recognise a male foetus as foreign and generate a strong immune response.
This response can apparently strengthen with each subsequent male pregnancy and may affect the way that the brain develops sexually.
This theory is supported by a substantial number of other studies but does not apply to having older sisters.
Sisters have no impact, and there is no effect on girls, as female foetuses do not provoke the same reaction.
Certain parts of the body are affected by the male sex hormone testosterone during the development of the foetus and clues from the shape of ears, fingers, eyes and arms all indicate that many lesbians are exposed to higher levels of testosterone in the womb.
For the study Dr. Bogaert surveyed 1,000 heterosexual and homosexual men in Canada who had either biological or non-biological brothers.
The research is published in the current issue of the journal Proceedings of the National Academy of Sciences.