Recent Research on Sexual Orientation and Fraternal Birth Order
Bogaert, Anthony F., The Canadian Journal of Human Sexuality
ABSTRACT: Number of older brothers (or "fraternal" birth order) predicts a homosexual orientation in men but not in women. In this paper, I review recent research on the fraternal birth order effect. For example, I present a recent study using two national probability samples that indicates that number of older brothers increases homosexual attraction but not homosexual behaviour/experience in men. In addition, I present a study using Canadian data indicating that fraternal birth order may interact with height to predict sexual orientation in men such that a homosexual orientation is most likely to occur in men who have a high number of older brothers and a shorter stature. Results of these and other recent studies are discussed in relation to biological and psychosocial theories of the fraternal birth order effect.
Key words: Sexual orientation Birth order
ACKNOWLEDGEMENTS: This research was supported by a Social Sciences and Humanities Research Council of Canada Grant (# 410-99-0521) to Anthony F. Bogaert. The author wishes to thank Ray Blanchard, Andre Bradley, John Cairney, Carolyn Hafer, Cathy Hopkins, Catharine Irwin, Sarah Rose, and Stan Sadava for their help at various stages of this research. This article is adapted from a paper presented at the Canadian Sex Research Forum meeting in Toronto, September 2002.
Although there is now a sizeable body of theoretical and empirical research on the development of sexual orientation, there is no single theory that can confidently predict how a person becomes gay, lesbian, bisexual, or heterosexual. Most of the research in this area has involved gay men, and less often lesbians and bisexuals, and has sought to identify social or biological factors that are found more often among gay than heterosexual subjects (e.g., Bailey et al., 2000; Bern, 1996; Ellis & Ames, 1987; Gangestad, Bailey, & Martin, 2000, LeVay, 1996; Singh, Vidaurri, Zambarano, & Dabbs, 1999; Williams et al., 2000). A number of such factors have been correlated with sexual orientation, including genetic factors, gender role behaviour in childhood, and certain aspects of physical development (for a recent review see Zucker, 2001). It should be emphasized that correlation does not mean causality but rather that such factors may reflect underlying mechanisms involved in psychosexual development. Furthermore, none of the factors identified to date is so strongly correlated that it "explains", in a statistical sense, how people get to be gay, lesbian, or bisexual. Among the factors cited above, childhood non-conformity with gender role expectations among boys has been among the best "predictors" of male homosexual orientation in adulthood and a number of explanations for this phenomenon have been advanced (Bern, 1996).
In the 1990s, birth order emerged in the literature as another factor that has shown a strong enough correlation with homosexual orientation in men to warrant attention (Blanchard, 1997). This paper reviews recent research on the relationship between birth order and sexual orientation and presents hypotheses concerning the mechanisms through which a male's birth position in the family might influence sexual orientation. Before doing so, a common question about this area of research should be addressed. Why study the development of sexual orientation and specifically same-sex sexual orientation?
WHY IT IS IMPORTANT TO STUDY SEXUAL ORIENTATION
Studying sexual orientation is important for a number of reasons. A person's sexual orientation (the erotic and affectional inclination toward the same, opposite, or both sexes) is a fundamental aspect of sexuality and identity. Study in this area is thus a basic aspect of basic sexological research. Given the differences in sexual orientation, such research also contributes to the study of human variation, personality and individual differences. …