A male is more likely to be gay if he has an older brother, the likelihood grows the more older brothers he has. The percentage of gay males is estimated to be around 3%, this probability can go up to 5% for males with several older brothers, say researchers from Brock University, St. Catharines, Canada.

The researchers are certain there is a biological basis for sexual orientation – in other words, there is a prenatal effect. It is not a case of older brothers having a psychological effect on the male baby after it is born. Males with older stepbrothers, or adopted brothers, are not more likely to be gay, only males with blood brothers. The scientists say the effect has to be through the mother, the only link between them.

You can read about this study in the Proceedings of the National Academy of Sciences.

Previous studies have shown a link between male homosexuality and the number of older brothers. This study is the first one to factor out social and environmental effects.

Study leader, Anthony Bogaert, and team examined four groups of men – 944 males. They looked at how many male and female siblings they had, whether they were blood related and lived in the same house when they grew up. The also looked at whether the men had been adopted.

They found that males with one older blood brother were more likely to be gay than males with no older brother(s). The more older brothers a male had the higher his chances of being gay. They said the likelihood had the same increase when blood brothers were raised in different households.

The team stressed that even with several older brothers, the chances of a male being heterosexual is 95%. 97% of all males are heterosexual.

Only males who had an older brother from the same mother had a higher chance of being gay, said the researchers.

The researchers said the environment the male was brought up in makes no difference at all. The only link is that the older brother(s) shared the same womb.

Proceedings of the National Academy of Sciences

http://www.pnas.org

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

http://www.sshrc.ca/web/home_e.asp

Written by: Christian Nordqvist

Editor: Medical News Today