The study was conducted by Jakub Fořt of Charles University in Prague, who analyzed data from an online survey of 3,963 respondents collected between June 2020 and March 2021 in Czechia and Slovakia. After a series of careful exclusions—removing participants who did not report their sex, were under 18 or over 79, were adopted or unsure about it, identified as transgender, or were classified as homosexual—the final sample comprised 843 heterosexual men, 184 bisexual men, 58 asexual men, 331 heterosexual women, 248 bisexual women, and 32 asexual women. Sexual orientation was assessed using five Kinsey-like items covering self-identification, fantasies, desires, and the sex of sexual partners over the past year and past five years, while asexuality was determined by self-identification alone.

The fraternal birth order effect was first documented by Ray Blanchard and Anthony Bogaert in 1996, and it has since been replicated by numerous research teams across different geographic regions, including in a very large dataset drawn from Dutch population-level administrative records. Meta-analyses have confirmed the pattern: each additional older brother increases the odds of male homosexuality by a modest but reliable margin. Crucially, the effect appears to depend on maternal siblings—boys who share a biological mother with their older brothers—rather than on adopted or stepbrothers, which points strongly toward a biological mechanism operating during pregnancy rather than a social or rearing influence.

The leading explanation is the maternal immune hypothesis. According to this framework, male fetuses carry proteins that are foreign to the mother’s immune system, most notably male-specific, Y-linked antigens. With each pregnancy carrying a male fetus, the mother’s immune system is progressively sensitized and produces antibodies against these antigens. In subsequent pregnancies, these antibodies can cross the placenta and may alter the sexual differentiation of the developing brain in ways that increase the likelihood of same-sex attraction in adulthood. The hypothesis received a major boost in 2018, when a laboratory study reported that mothers of gay men had elevated blood levels of antibodies against neuroligin 4 Y-linked, or NLGN4Y, a male-specific protein, compared with mothers of heterosexual men.

A complementary line of thinking involves fetal microchimerism, the phenomenon by which fetal cells cross the placenta, engraft in maternal tissues, and persist in the mother’s body for decades after birth. Male fetal progenitor cells have been detected in maternal blood as long as 27 years postpartum. These engrafted cells could, in principle, influence maternal physiology during later pregnancies or contribute to a sustained production of antibodies, providing a plausible route through which the birth order effect could operate. The two mechanisms are not mutually exclusive, and researchers note that both remain active areas of investigation.

In the new study, Fořt employed modern statistical methods designed to disentangle the fraternal birth order effect from a separate phenomenon known as the female fecundity effect, in which mothers of gay men have been reported to have more children overall. Because larger sibships mechanically increase the probability of having more older brothers and sisters, conflating the two effects has been a persistent methodological problem. The analysis therefore used two logistic regression parameterizations—one treating sibship size, number of older siblings, older brothers, and younger brothers as predictors, and a parallel version substituting older and younger sisters—while controlling for the participant’s age and the mother’s age at the participant’s birth to account for birth cohort effects. The study also verified that sibling sex ratios in the heterosexual reference groups did not deviate from the expected population value of 106 boys per 100 girls, guarding against distortions from parental stopping rules.

The headline result was clear: the number of older siblings significantly increased the odds of bisexuality in men, with an odds ratio of 1.66, meaning each additional older sibling raised the probability of male bisexuality by roughly two-thirds. Notably, this effect size is almost identical to that previously found for homosexual men in the same Czech and Slovak sample, and it is somewhat larger than earlier meta-analytic estimates for gay men drawn from convenience and probability samples. No statistically significant effects emerged for bisexual women, asexual men, or asexual women, and no significant sororal birth order effect—concerning older sisters—or sibling size effects were detected in any group.

Yet the pattern of non-significant results was itself intriguing. For bisexual women, asexual men, and asexual women, the estimated fraternal birth order effect pointed in the same direction and was of a magnitude broadly similar to that observed in bisexual men, with odds ratios of 1.46, 1.70, and 1.62 respectively. The failure to reach statistical significance likely reflects limited statistical power, particularly given the small numbers of asexual participants—only 58 asexual men and 32 asexual women were included. Post-hoc power analyses indicated acceptable power only for bisexual men, with power falling to 0.68 for bisexual women and just 0.46 for asexual women. The author cautions, therefore, that these findings should be treated as preliminary until larger, more diverse samples are examined.

If the fraternal birth order effect in bisexual men is corroborated by future studies and meta-analyses, the implication would be significant: whatever biological mechanism underlies the effect, it appears to be shared between exclusively and partially same-sex attracted men. In other words, male bisexuality and male homosexuality may share a common biodevelopmental origin, rather than arising through entirely distinct pathways. The author even speculates that the mechanism could extend across mammalian species, noting a recent study in which male laboratory rats born to multiparous mothers showed a greater tendency toward same-sex partner preference—a fraternal birth order-like pattern outside humans.

The findings also contribute to a sparse literature on asexuality, a sexual orientation that has been almost entirely absent from birth order research until recently. Prior evidence has been mixed: an early study suggested asexual men had more older brothers than heterosexual men, but subsequent rigorous analyses found no such effect, and results on sibship size among asexual individuals have been contradictory. One recent study of UK Biobank participants reported smaller sibship sizes for asexual men and women, while another found larger sibships for asexual men and no difference for asexual women. The present study adds another data point, though its own limitations—including the convenience sampling method, the online format, and the modest asexual subsample—mean that definitive conclusions about asexuality and sibling composition remain out of reach for now.

Subject of Research: The fraternal birth order effect and other sibship effects in bisexual and asexual men and women

Article Title: Fraternal Birth Order Effect and Other Sibship Effects in Bisexual and Asexual Men and Women

Article References: Fořt, J. (2026). Fraternal Birth Order Effect and Other Sibship Effects in Bisexual and Asexual Men and Women. Archives of Sexual Behavior. https://doi.org/10.1007/s10508-026-03536-9

Image Credits: AI Generated

DOI: 10.1007/s10508-026-03536-9

Keywords: fraternal birth order effect, sexual orientation, bisexuality, asexuality, maternal immune hypothesis, birth order, sibship size, fetal microchimerism, female fecundity effect, Archives of Sexual Behavior, NLGN4Y, evolutionary biology