Scientists have documented new evidence that men with older brothers have an increased chance of being gay, according to CNN.
CNN on Monday reported that the phenomenon has been observed in previous research from scientists.
New results published on Monday in the Proceedings of the National Academy of Sciences of the United States of America journal, however, shed new light on the topic.
Scientists now believe they have a biological explanation for why the pattern exists, adding it starts long before birth.
The researchers claim that if their findings can be duplicated, humans will understand at least one of the biological reasons some men are born gay.
Multiple factors influence a person’s sexual orientation, but researchers detected a pattern in this instance that may be tied to something that occurs in a mother’s womb.
The event is related to a protein associated with the Y chromosome – which women lack – that is vital for male brain development.
Researchers think it’s possible that when a woman gets pregnant with her first boy, this Y-lined protein enters her bloodstream.
The mother’s body then recognizes the protein as a foreign substance, prompting her immune system to create antibodies.
The antibodies can ultimately build up in the woman’s body to a point where they cross the placental barrier once she gets pregnant with another boy, entering the second child’s brain.
“That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction,” said Anthony Bogaert, a Canadian professor and psychologist who is the study’s author.
Bogaert works in the departments of psychology and community health sciences at Brock University in St. Catharines, Ontario, Canada.
The Canadian researcher and his co-authors tested 142 women and 12 men ages 18 to 80 for the study.
The study found that women had a higher concentration of the antibodies to the protein – called NLGN4Y – in blood samples than did men.
The highest concentration of NLGN4Y came from women with gay younger sons who had older brothers.
The concentrations were compared to those from women with no sons or those who had only birthed heterosexual boys instead.
Past research suggests that the more older brothers a boy has, the more his chances that he will find men attractive.
A 2006 study found that with each brother, the chance rises by about one third, but researchers did not determine why that was.