Fathers’ Facial Dominance Predicts First-Born Sons in Parent Dyads

The Trivers-Willard hypothesis (TWH) states that offspring sex should vary depending on parent condition, and TWH effects have been studied extensively. Findings have been equivocal, however, and recent work has challenged the TWH’s theoretical predictions. One possible reason for variation in TWH findings is that few studies have investigated effects of mate selection for condition on offspring sex. Here we tested whether more dominant parents (N = 104 dyads from Prolific) would be more likely to share a first-born son than a first-born daughter.

Parent couples completed a survey of family demographics and dominance measures then submitted facial photographs. Photographs were standardized and rated by undergraduates for perceived facial dominance. Facial width-to-height ratio (fWHR) was also measured.

We found that rated paternal facial dominance, but not rated maternal facial dominance or their interaction, predicted the likelihood of having a first-born son. Self-reported dominance was not a reliable predictor of offspring sex, and fWHR did not predict OSR.

These results suggest that fathers’ facial dominance might influence the likelihood of a couple producing male offspring. We propose a plausible mechanism through which maternal personality, hormones, and mate preferences influence the sex of offspring. Relationships between facial cues of dominance and offspring sex warrant further investigation.