While the data did not contain direct measures of individuals' sexual orientation, they did indicate whether they ever entered a same-sex marriage or registered partnership. This time, we used it to provide a robust test of the fraternal birth order effect. In previous studies we used this dataset to examine whether the gender of a married couple's children affected the stability of their union, and to compare the academic performance of children raised by same- and different-sex couples. These data allowed us to follow the life trajectories of more than nine million people born between 19. Our research used unique data from Dutch population registers. Indeed, no study of a representative population sample has supported its existence-until now. However, previous research documenting the fraternal birth order effect has relied on small and selective participant samples, which has led some scholars to question the authenticity of the phenomenon. These maternal antibodies accumulate over successive pregnancies with male fetuses, which means men with more older brothers are more likely to experience same-sex sexual attraction. The proteins enter the mother's bloodstream and trigger the production of antibodies that influence the sexual development of subsequent children.
This effect has been attributed to a mother's immune reaction to proteins produced by a male fetus. This longstanding hypothesis proposes men's propensity for homosexuality increases with the number of older biological brothers they have. The "fraternal birth order effect" is one of the most well-documented patterns supporting a biological origin of human sexual orientation. These studies have substantial implications for public opinion and debate, and subsequently the treatment of LGBTIQ+ people.įor example, we know people who view sexual orientation as a product of biological factors (such as hormones or genetics) are more likely to support sexual minorities and their civil rights, compared to those who view it as a product of social factors or individual choice. But despite these encouraging developments, sexual minorities still experience high levels of stigma-and the origins of sexual orientation remain a matter of debate.Ī growing body of research is attempting to shed light on why some people experience same-sex sexual attraction and others don't. In recent decades, many countries have achieved remarkable progress towards equal treatment of LGBTIQ+ people, including greater public support and more protective legislation. This finding, detailed in our paper published today in the Journal of Sex Research, offers a rare insight into the origins of sexual orientation.