(CNN) - If you're a guy with an older brother, there's an increased chance you're gay.
Scientists have noticed this pattern in previous research, but now they think they have a biological explanation as to why, and it starts long before birth. The results were published in the journal PNAS on Monday.
The researchers say that if their findings can be replicated, we may know at least one of the biological reasons some men are gay.
Many factors may determine someone's sexual orientation, but in this case, researchers noticed a pattern that may be linked to something that happens in the womb. The phenomenon is related to a protein linked to the Y chromosome (which women do not have) that is important to male brain development.
Researchers think it's possible that when a woman gets pregnant with her first boy, this Y-linked protein gets into her bloodstream. The mother's body recognizes the protein as a foreign substance, and her immune system responds, creating antibodies. If enough of these antibodies build up in the woman's body and she gets pregnant with another a boy, they can cross the placental barrier and enter the brain of the second male fetus.
"That may alter the functions in the brain, changing the direction of how the male fetus may later develop their sense of attraction," said study author Anthony Bogaert, a Canadian psychologist and professor in the departments of psychology and community health sciences at Brock University.
Earlier research has shown that the more older brothers a boy has, the more of a chance that boy will be attracted to men. A 2006 study showed that with each brother, the chance that a man will be gay goes up by about a third, but the researchers didn't determine why that was.
Bogaert and his co-authors tested a small group of 142 women and 12 men ages 18 to 80 and found a higher concentration of the protein, known as NLGN4Y, in blood samples from women than from men. They found the highest concentration of the protein in women with gay younger sons who had older brothers, compared with women who had no sons or who had given birth to only heterosexual boys.
The study builds on research Bogaert and his co-authors have been exploring for more than 20 years. Since their initial research that noted the trend, other research -- although not all studies -- have detected the phenomenon, even across cultures. One found that a man's chances of being gay increased even if he was raised apart from his older brother.
Researchers did not see a similar pattern in families with adopted brothers, so scientists started to think there must be a maternal developmental explanation. The research does not give a biological explanation for why some men may be bisexual or may not be attracted to anyone at all, nor can it give a biological explanation for gay only children, gay oldest sons or women who are attracted to women.
J. Michael Bailey, a professor in the Department of Psychology at Northwestern University, thinks the latest research is important. "It is significant, and I believe science granting agencies should put a high priority into additional research to see if this is true," he said.
Bailey was not involved in the new study but has worked on studies that have found genetic factors that may explain some differences in sexual orientations.
Bailey's latest paper, published this month in the journal Nature Research, looked at people's genomes and found several regions with single-letter DNA changes that were more common among gay men than straight men and may be relevant to the development of sexual orientation. Bailey believes this new study may be even more significant than general genetic findings if the findings can be replicated.
"Our studies only show that there may be genes that matter in sexual orientation," he said. "It is not like this study, that shows there is a potential specific mechanism by which sexual orientation may have changed prenatally. This is important work and fascinating if it proves to be true."
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