Reconsidering the Scope and Mechanisms of the Birth Order Effect on Sexual Orientation

Large-scale research in the past few years has confirmed the many previous findings that individuals with a greater number of older siblings (i.e., a higher birth order) have a greater probability of homosexuality. The birth order effect is now among the better-established empirical findings in research on human homosexuality. These studies have, at the same time, challenged the conclusion that the birth order effect is caused solely by older brothers and that its impact is limited to younger brothers. Rather, the available evidence now suggests that females are also, and perhaps equally, involved in the phenomenon. The newer findings, therefore, necessitate reconsideration of the hypothesis that the underlying mechanism involves a straightforward maternal immune response to male-specific (i.e., Y-chromosome-linked) fetal antigen. The present document argues that a maternal immune response of some kind nevertheless remains the most likely explanation of the birth order effect and suggests testable modified interpretations of the data.