Approved Research
Fraternal Birth Order in Bisexual, Homosexual, Heterosexual, and Asexual Men and Women
Lay summary
In recent decades, many countries have achieved remarkable progress towards equal treatment of LGBTIQ+ people, including greater public support and more protective legislation. But despite these encouraging developments, sexual minorities still experience high levels of stigma and prejudice. The negative perception of sexual minorities is often associated with the belief that alternative sexual orientations are a product of "lifestyle choices" or "fashion trends" rather than innate traits that reflect individuals' true selves.
To this end, research into the biological origins of sexual orientation can constitute a powerful tool for informing the public debate. One of the most established theories underlying the biological origins of human sexual orientation is the fraternal birth order effect (FBOE), which shows that older male siblings increase the odds of homosexuality in later-born males.
While the existence of the FBOE has been demonstrated in a multitude of different contexts, there are several outstanding research questions that could greatly enhance the practical relevance of this scholarship. One is whether older brothers increase the odds of homosexuality in later-born females. Another is whether older brothers increase the odds of bisexuality in later-born males and females. If confirmed, each of these findings would make the FBOE phenomenon much more inclusive, shedding light on the biological mechanisms underlying female homosexuality and bisexuality in general.
Our analysis will draw on the questionnaire form from the UK Biobank, which contains the required information on respondents' sexual orientation and sibship characteristics. Statistical analysis will use the logistic regression models, with the criterion variable being sexual orientation, dichotomously coded as homosexual or heterosexual, and the predictor variables being total number of siblings, number of older siblings, number of older brothers, and number of younger brothers.
Our project is expected to take 1.5 years to complete, and it is likely to be of interest to both the scientific community and the general public at large. This is because the proposed research tackles fundamental questions regarding the biological origins of human sexual orientation. Previous research has shown that members of the public have more positive attitudes towards sexual minorities if they believe that sexual attraction has biological roots. As such, academic studies in this space can have substantive positive social implications, enhancing the wellbeing and perception of sexual minorities. The impacts on various psychosocial and mental health outcomes are likely to be particularly prominent.