CNN If you're a guy with an older brother, there's an increased chance you're gay. Clarification: A previous version of this story referred to higher concentrations of the protein instead of higher concentrations of antibodies to the proteins when describing the findings of the study. More Videos The science of sexuality Story highlights Scientists say mothers who have more than one boy had higher concentrations of a certain protein Earlier studies have noticed that gay men often have older brothers.
Study suggests males with older brother more likely to be gay
Homosexuality in men and number of older brothers
Scientists may have discovered why the more older brothers a boy has, the likelier he is to be gay as an adult. Antibodies are proteins produced by the immune system to fight harmful bacteria and viruses. But pregnant women sometimes produce antibodies against fetal molecules, such as when their foetus has a different blood group. Anthony Bogaert, a professor in social psychology at Brock University, Canada , believes that some women who are pregnant with boys produce antibodies that neutralise a protein made by the Y chromosome; the DNA molecule that determines sex in humans.
If You’re a Guy, Having Older Brothers Makes You More Likely to Be Gay
December 12, Groundbreaking research led by a team from Brock University has further confirmed that sexual orientation for men is likely determined in the womb. In the first-ever laboratory study of mothers of gay men, the research was prompted by more than two decades of statistical data examining the 'older brother effect' which shows that biological older brothers—but not older sisters—increase the odds of homosexuality in later-born males. Brock Health Sciences Professor Tony Bogaert, lead researcher on the project, said the new study has produced some of the most significant findings in men's sexual orientation research in the past 10 or 15 years. The team included researchers from Harvard and the University of Toronto.
Objective: This study investigated whether homosexual men have a higher mean birth order than heterosexual men primarily because they have more older brothers or because they have more older siblings of both sexes. Method: For the main analyses, heterosexual men were individually matched on year of birth with an equal number of heterosexual men. Each completed a self-administered, anonymous questionnaire concerning family background and other biodemographic information. Results: Logistic regression analysis showed that homosexuality was positively correlated with the proband's number of older brothers but not with older sisters, younger brothers, younger sisters, or parental age at the time of the proband's birth. Conclusions: These results restrict the range of possible theories of the birth order phenomenon to those that can explain not only why older brothers increase the probability of homosexuality in later-born males but also why older sisters neither enhance this effect nor counteract it.