Biological versus nonbiological older brothers and men’s sexual orientation

Source: PNAS. Proceedings of the National Academy of Sciences
11 July 2006

Biological versus nonbiological older brothers and men’s sexual orientation

Anthony F. Bogaert*

Departments of Community Health Sciences and Psychology, Brock University, St. Catharines, ON, Canada L2S 3A1

Edited by Dale Purves, Duke University Medical Center, Durham, NC, and approved May 17, 2006 (received for review December 29, 2005)

ABSTRACT

The most consistent biodemographic correlate of sexual orientation in men is the number of older brothers (fraternal birth order). The mechanism underlying this effect remains unknown. In this article, I provide a direct test pitting prenatal against postnatal (e.g., social/rearing) mechanisms. Four samples of homosexual and heterosexual men (total n = 944), including one sample of men raised in nonbiological and blended families (e.g., raised with half- or step-siblings or as adoptees) were studied. Only biological older brothers, and not any other sibling characteristic, including nonbiological older brothers, predicted men’s sexual orientation, regardless of the amount of time reared with these siblings. These results strongly suggest a prenatal origin to the fraternal birth-order effect.

* * *

Recent research has provided evidence that genetic and prenatal factors may influence sexual orientation development (1–7). In this article, I demonstrate that the number of biological older brothers, including those not reared with the participant (but not the number of nonbiological older brothers), increases the probability of homosexuality in men. These results provide evidence that a prenatal mechanism(s), and not social and/or rearing factors, affects men’s sexual orientation development.

The most consistent biodemographic correlate of sexual orientation in men is the number of older brothers, originally observed by Blanchard and Bogaert (8) in a Canadian sample in the 1990s but since then found in samples from different eras and from different countries, both by us and independent investigators (2, 7, 9–12). Evidence does not exist that sibling characteristics reliably correlate with women’s sexual orientation (13, 14). Both childhood social/rearing (13, 15, 16) and prenatal (8, 13) mechanisms have been advanced to account for the older brother (“fraternal birth-order”) effect in men, but a direct test pitting prenatal versus postnatal (e.g., social/rearing) mechanisms is lacking. Such a test is possible when information on both biological and nonbiological siblings, along with sexual orientation, is included in the research design.

Four samples of homosexual and heterosexual men (total n = 944) reporting on their parental and sibling characteristics (i.e., biological and nonbiological siblings) were examined to test this issue. Three samples were archival and contained men with (largely) biologically intact families. These samples contained information on all siblings (both biological and nonbiological) with whom the participant was reared. The fourth sample was recruited specifically to test the research issue investigated in this article and contained men with nonbiological or blended families (e.g., raised with half- or step-siblings or as adoptees). This final sample also contained information on the amount of time the participants were reared with each sibling, along with information on any biological siblings with whom they were never reared.

If rearing or social factors associated with older male siblings underlies the fraternal birth-order effect, then all older brothers reared with the participant should predict sexual orientation because all of these older male siblings (both biological and nonbiological) share the social/rearing environment with their younger male siblings. If a prenatal factor underlies the fraternal birth-order effect, however, then only biological older brothers should predict sexual orientation because only biological older brothers (and not nonbiological older brothers) share prenatal characteristics (e.g., gestated by the same biological mother) with their younger male siblings. Second, if rearing or social factors underlie the fraternal birth-order effect, then the amount of time reared with older brothers, either biological or nonbiological, should predict sexual orientation because rearing time indexes the relative opportunity that older brothers have to affect their younger sibling’s (postnatal) sociosexual development. If a prenatal factor underlies the fraternal birth-order effect, however, then a postnatal factor such as rearing time with older siblings (be they biological or nonbiological) should have no impact on the sexual orientation of younger male siblings. Finally, if rearing or social factors underlie the fraternal birth-order effect, then the number of biological older brothers with whom the participants were not reared should not predict sexual orientation because they should have no impact on the (postnatal) sociosexual environment of their younger brothers. If a prenatal factor underlies the fraternal birth-order effect, however, then biological older brothers with whom the participants were not reared should predict sexual orientation because all biological older brothers, even those not reared with the participants, share prenatal characteristics (e.g., gestated by the same mother) with their young male siblings.

Editor’s note: Only the abstract is archived here. For the full article, go to the source link as given at the top of the page.

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