The probability of a man being gay rises the more older brothers he has, a finding that adds weight to the idea sexual orientation could be influenced in the womb.

In 1997, a study led by Ray Blanchard of the Centre for Addiction and Mental Health in Toronto, concluded that having older brothers increases the likelihood of homosexuality in younger brothers.

To find out if psychological factors in the family were behind the effect, Anthony Bogaert of Brock University in St. Catharines compared men with older brothers born to the same mother to those whose older brothers were adopted or stepbrothers.

If nurture was responsible for the birth order effect then both groups would be expected to show similar rates of homosexuality.

Instead, Bogaert's study of 944 men found only biological older brothers showed an effect on sexual orientation, whether or not they were raised together.

The findings are reported in Tuesday's issue of Proceedings of the National Academy of Sciences.

The effect was seen with one older brother, and seemed to be stronger for those with three or more older brothers, Bogaert said.

The overall rate of homosexuality in men is about three per cent, Bogaert estimated.

Although having several older brothers may increase the rate to five per cent, the remaining 95 per cent of men with several older brothers are heterosexual, he said.

Some sort of "prenatal factor" such as a maternal immune response to several male fetuses could be at play, altering the brain development of younger brothers, Blanchard told CBC Newsworld.  

The findings don't rule out the effects of learning in homosexuality, but more research is generally finding prenatal effects, he said.

The research was funded by the Social Sciences and Humanities Research Council of Canada.