Fraternal birth order effect on sexual orientation explained

Abstract

Twenty years ago, Ray Blanchard and Anthony Bogaert demonstrated that the probability of a boy growing up to be gay increases for each older brother born to the same mother, the so-called fraternal birth order (FBO) effect. Their first investigation indicated that each older brother increased the probability of being gay by about 33% (1). This startling phenomenon was confirmed in multiple studies based on independent populations totaling over 10,000 subjects, and a meta-analysis indicated that between 15% and 29% of gay men owe their sexual orientation to this effect (2). Despite this compelling evidence, a mechanism to account for the effect remained elusive. In PNAS, Bogaert et al. (3) present direct biochemical evidence indicating that the increased incidence of homosexuality in males with older brothers results from a progressive immunization of the mother against a male-specific cell-adhesion protein that plays a key role in cell–cell interactions, specifically in the process of synapse formation, during development called neuroligin 4 Y-linked, or NLGN4Y. This study provides the first data-based explanation for the FBO effect and adds a significant chapter to growing evidence indicating that sexual orientation is heavily influenced by prenatal biological mechanisms rather than by unidentified factors in socialization. The nature–nurture debate still rages in the minds of many scientists and scholars, despite the consensus that these are complementary rather than mutually exclusive explanations. However, no field of research subject to this debate generates more heated controversies than those probing the proximate causes of sexual orientation, particularly its less-frequent and thus, perhaps, more-perplexing form: homosexuality. Why the obverse questions probing the causes of heterosexuality attracts no attention remains enigmatic. Theories relying mainly on psychological and social mechanisms contend that the newborn is essentially neutral and that sexual orientation develops during infancy and childhood through a variety of socializing influences. In … [↵][1]1Email: jbalthazart{at}uliege.be. [1]: #xref-corresp-1-1