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Reassessing the Effect of Older Sisters on Sexual Orientation in Men

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Abstract

This research reanalyzed questionnaire data from 8279 homosexual and 79,519 heterosexual men who participated in 2005 in an internet-based research project sponsored by the British Broadcasting Corporation. It focused on parameters of sibship composition (older brothers, older sisters, younger siblings) previously shown or hypothesized to influence sexual orientation in males. The results included the usual finding that older brothers increase the odds of homosexuality in later-born males. As predicted, older sisters also increase those odds, although by a lesser amount than older brothers. Other results confirmed that the odds of homosexuality are increased in only-children, the amount of increase being equal to that produced by one older brother and greater than that produced by one older sister. Finally, the results indicated that younger siblings have no effect on the odds of homosexuality in males. These results might be explained by the hypothesis that two different types of immune responses in pregnant women can affect the future sexual orientation of their male fetuses. One type of response affects fetuses in first pregnancies and reduces subsequent fertility. The other type affects fetuses in later pregnancies and has little or no effect on fertility. Finally, we conducted an estimate of combined sibship effects. Men who were exposed to any of the influences that we identified (being an only-child or having an older sibling) had 27% greater odds of homosexuality than did subjects who were exposed to none of these influences (i.e., the first-born of two or more children).

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Notes

  1. One would also expect the effects of liveborn older brothers to be slightly augmented by the effects of occult male miscarriages, but the effects of the latter would be overshadowed by those of the former.

  2. This finding has subsequently been confirmed by Blanchard et al. (2020a).

  3. The two isoforms of NLGN4Y were tested separately, in different trays, at different times, and—most importantly—with independently created antigens. Thus, the general similarity in the results obtained with isoform 1 and isoform 2 can be seen as a kind of internal replication.

  4. The research design of comparing first-born homosexual men and first-born heterosexual men with regard to their mean numbers of younger siblings was introduced by Camperio-Ciani, Corna, and Capiluppi (2004) and subsequently incorporated by Khovanova (2020) into her method. Since differences in birth order (fraternal or otherwise) are eliminated by this procedure, any differences in sibship size (barring religious, socioeconomic, ethnic, or other demographic confounds) can reasonably be attributed to differences in parental fertility. Younger siblings are taken as a proxy for maternal fertility, not because they necessarily carry superior information about the mother’s willingness or capacity to reproduce, but because they simplify statistical analyses and their interpretation.

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Authors and Affiliations

  1. Department of Psychiatry, University of Toronto, Toronto, ON, M5T 1R8, Canada

    Ray Blanchard

  2. Department of Psychology, California State University, Fullerton, CA, USA

    Richard A. Lippa

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  1. Ray Blanchard
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  2. Richard A. Lippa
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Correspondence to Ray Blanchard.

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This study was carried out on archived data. Institutional ethics approvals were obtained by the original investigators (Reimers, 2007).

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Blanchard, R., Lippa, R.A. Reassessing the Effect of Older Sisters on Sexual Orientation in Men. Arch Sex Behav 50, 797–805 (2021). https://doi.org/10.1007/s10508-020-01840-6

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  • DOI: https://doi.org/10.1007/s10508-020-01840-6

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