How consistent is the fraternal birth‑order effect across non‑Western populations, and what cultural or biological factors could explain differences?
Executive summary
The fraternal birth‑order effect (FBOE)—the finding that each additional biological older brother raises the odds a man will be androphilic—has been replicated in many non‑Western settings but not uniformly, with effect sizes modest and heterogeneous across populations [1] [2]. Leading biological explanations, notably the maternal immune hypothesis, offer a cross‑cultural mechanism, while cultural, demographic and methodological factors help explain observed differences between studies [1] [3].
1. What the global data actually show: replication with caveats
Meta‑analyses and reviews report FBOE findings in diverse nations including Brazil, Turkey, Samoa and Iran, as well as in North America and Europe, indicating the effect is not exclusive to Western samples [2] [4]. Samoa has been a prominent non‑Western replication: studies found androphilic fa'afafine with more older brothers (and in some analyses more older and younger brothers) than gynephilic males, supporting both FBOE and maternal fecundity effects [5] [6]. Yet not all non‑Western work is consistent—some populations yield mixed or smaller effects, and preregistered work in East Europe and some East Asian samples has produced complex patterns that sometimes implicate sororal effects or interact with family‑size differences [7] [8].
2. How big is the effect and why that matters across cultures
The FBOE is robust statistically but modest in magnitude—each older brother increases odds of male androphilia roughly 20–35% above baseline in many reports—so population fertility and family‑size variation materially change how visible the effect is in a given society [8] [3]. High‑fertility societies amplify the demographic imprint of FBOE on population prevalence, whereas low‑fertility or small‑family norms make the signal harder to detect and more sensitive to sampling error [3]. Consequently, cross‑cultural comparisons must control for fertility, sibship distributions and sampling frames or risk apparent inconsistencies that are actually demographic artifacts [3].
3. Biological explanation favored across cultures: maternal immune hypothesis
The dominant biological model—the maternal immune hypothesis—posits that progressively stronger maternal antibodies to male‑specific antigens during successive male pregnancies alter neurodevelopment in later‑born males, a mechanism that would operate irrespective of culture and accords with evidence that only biological older brothers matter (not stepbrothers) [1] [9]. Studies citing immune responses to Y‑linked proteins and biodemographic patterns lend support to a prenatal biological pathway that can plausibly produce similar effects worldwide, which explains why replicated signals are found in both Western and some non‑Western samples [1] [8].
4. Cultural and methodological factors that produce variability
Cultural variables—such as recognition of third gender categories (e.g., fa'afafine in Samoa), norms about family composition, reporting biases, and differential recruitment of visible androphilic men—affect sample composition and the detectability of FBOE; for example, public presentation patterns among fa'afafine can influence which men enter studies [5] [8]. Methodological heterogeneity—small samples, non‑probability recruitment, lack of controls for family size or handedness, and potential statistical artifacts flagged in recent critiques—further explain mixed findings across non‑Western studies and caution against overgeneralization [10] [9].
5. Where evidence is thin and controversies remain
Although many studies support FBOE cross‑culturally, research remains uneven: some regions lack high‑quality, preregistered, population‑representative studies, and debates persist about sororal birth‑order effects, interaction with handedness, and whether any statistical artifacts could account for parts of the literature [7] [9] [10]. Reviewers and newer analyses call for more cross‑cultural comparisons that explicitly model fertility, maternal fecundity, and sampling biases to separate true biological universals from demographic or methodological confounds [11] [2].
Conclusion: a tentative consensus with important limits
The cautious conclusion is that FBOE appears across multiple non‑Western populations and is plausibly rooted in prenatal maternal biology, yet its expression varies by demographic context and study design; cultural categories and sampling practices can magnify or mask the effect, and unresolved methodological critiques mean further rigorous cross‑cultural work is needed before declaring a universal consistency [2] [1] [10].