Evidence of a Fraternal Birth Order Effect on Male and Female Same-Sex Marriage in the Dutch Population: A Reply to Blanchard and Semenyna, Gómez Jiménez & Vasey

A recent study by Christine Ablaza, Jan Kabátek and Francisco Perales published in Journal of Sex Research leveraged population-level administrative data from the Netherlands to generate novel evidence on the Fraternal Birth Order Effect (FBOE) and Female Fecundity Effect (FFE) on male and female homosexuality. Ray Blanchard, Scott Semenyna, Francisco Gómez Jiménez, and Paul Vasey formulated insightful remarks about the study. In this reply, we briefly respond to key points raised in their invited commentaries. First, we ascertain the degree to which endogenous stopping rules are present in the Dutch administrative data and discuss how such rules may influence the results of FBOE and FFE analyses in this and other datasets. Second, we comment on the value of population representativeness for this research, the relative strengths and weaknesses of same-sex marriage as an indicator of same-sex-oriented sexuality, and the desirability of incorporating sexual-orientation information in administrative datasets. Third, we summarize competing views on the meaning of an FBOE among women and outline how future studies could leverage administrative and other data sources to further investigate this phenomenon. We hope that these discussions invigorate research on the causes of human sexual orientation by pointing to avenues for theoretical and methodological refinement.