Surprising connection found between asexuality and family structure

A new study published in Archives of Sexual Behavior has found that patterns in family structure, particularly the number and gender of siblings, may be associated with a person’s likelihood of identifying as asexual. Researchers observed that men with more siblings were more likely to be asexual, while women who had fewer older sisters or were only children were also more likely to report asexuality. These findings suggest that some biological or social family factors could play a role in the development of sexual orientation, including asexuality.

Asexuality refers to a sexual orientation characterized by a lack of sexual attraction to others. Although it is now widely recognized as a legitimate sexual orientation, it remains less studied than others such as heterosexuality, homosexuality, and bisexuality. To better understand the development of asexuality, researchers have begun to explore whether biological or familial patterns—such as sibling order and family size—might be associated with asexual identity. These patterns have previously been linked to other sexual orientations, particularly same-sex attraction in men, which has been associated with the number of older brothers a person has, a pattern known as the fraternal birth order effect.

In the current study, the researchers aimed to explore whether similar sibling-related patterns also apply to asexuality. They recruited a large, international sample of 1,634 adults who identified as heterosexual, asexual, bisexual, or homosexual. The sample included 366 asexual participants, 276 who identified as gay or lesbian, 267 who identified as bisexual, and 725 who identified as heterosexual. Participants completed online questionnaires that assessed their sexual orientation, romantic and sexual history, and detailed information about their siblings—including the number of older and younger brothers and sisters.

To analyze the data, the researchers used two advanced statistical approaches designed to separate the effects of having more siblings from the specific effects of being later-born or having siblings of a particular sex. This distinction is important because having more siblings in general might have different implications than, for example, having more older brothers or sisters. These methods allowed the researchers to evaluate whether birth order and sibling gender were uniquely associated with different sexual orientations.

The study uncovered several notable findings related to asexuality. Among men, a higher total number of siblings was associated with an increased likelihood of identifying as asexual. This finding is consistent with what some researchers have described as the female fecundity effect, which refers to the idea that genetic traits linked to non-heterosexual orientations in men may also be linked to higher fertility in their female relatives. Although this theory has been debated and inconsistently supported in other research, the current findings raise the possibility that a similar pattern might apply to asexual men.

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For women, a different pattern emerged. Asexual women were more likely to be only children compared to heterosexual women, and they were also more likely to have fewer older sisters. These patterns were statistically significant and remained consistent across different methods of analysis. The researchers speculated that these results might reflect some kind of biological mechanism affecting early development, although no specific explanation has yet been confirmed. Another possibility is that the absence of older sisters may reduce exposure to certain social experiences or role models that influence sexual development and identity, suggesting a social rather than biological explanation.

Interestingly, the study did not find strong support for the fraternal birth order effect among asexual men. This effect, which has been observed in many studies of gay men, refers to the tendency for men with more older brothers to be more likely to identify as gay. In the current study, this pattern did not appear among asexual participants, suggesting that the mechanisms behind same-sex attraction and asexuality might be different.

The study also tested whether these sibling effects applied to individuals who identified as gay, lesbian, or bisexual. Among gay men, having more older sisters was associated with an increased likelihood of being gay—a finding known as the sororal birth order effect. The fraternal birth order effect was not significant using the primary analysis method but did reach significance using an alternative statistical approach on a smaller subsample. Among bisexual men, having more siblings in general was associated with an increased likelihood of identifying as bisexual, echoing the pattern found in asexual men. Among bisexual women, fewer older sisters were again associated with increased likelihood of bisexuality, similar to the pattern observed in asexual women.

The researchers noted several limitations to their study. The sample size for certain groups—such as asexual men and lesbians—was smaller than intended, which may have reduced the ability to detect some effects. The sample was also predominantly White and drawn from a wide range of countries and cultural backgrounds, which adds diversity but also increases variability that may obscure patterns. In addition, many participants with gender-diverse identities were excluded from the main analysis due to the study’s focus on biological sex and its hypothesized role in sibling-related effects. This decision may limit how broadly the findings apply to the asexual community, which includes many gender-diverse individuals.

Despite these limitations, the study provides one of the most comprehensive examinations to date of sibling composition in relation to asexuality. The findings suggest that asexuality, like other sexual orientations, may be shaped in part by early developmental or familial factors. However, the authors caution against drawing firm conclusions about causes. The associations observed in this study do not prove that sibling composition directly influences sexual orientation. Instead, these patterns offer clues that future research can investigate more deeply.

In particular, the researchers encourage further studies to replicate these findings and to explore possible biological or social mechanisms behind them. Including larger and more diverse samples, as well as participants with non-cisgender identities, will be important for building a fuller understanding of how asexuality develops.

The study, “Asexuality: Its Relationship to Sibling Sex Composition and Birth Order,” was authored by Bozena Zdaniuk, Sonia Milani, Brett Makarenko, Nicola Marriott, Anthony F. Bogaert, and Lori A. Brotto.