Biological versus nonbiological older brothers and men’s sexual orientation

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., social͞rearing) 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 halfor 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 birthorder effect.immune ͉ sexuality R ecent 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 and͞or 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 social͞rearing (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., social͞rearing) 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 ϭ 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 m...

Efectos mnemónicos maternos prenatales sobre el sexo psíquico humano. Insuficiencia del mecanismo inmunitario: Nueva propuesta desde la tolerancia-rechazo materno-fetal

“…Los datos detallados, análisis estadísticos e interpretación de los autores están en sus trabajos originales [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] , aquí presentamos nuestra interpretación basada en: 1) resultados de los sistemas ABO, Rh y HLA en las relaciones toleranciarechazo materno-fetales 19-26,31,38,39 y 2) la nueva proposición sobre la definición y ontogenia del sexo psíquico y social 1,2,12 .…”

“…Ellos suponen que la orientación sexual es determinada prenatalmente [3][4][5][6][7][8][9][10][11] . Esto es insostenible.…”

“…Estudios concluyentes indican que la orientación homosexual masculina es producida, en cerca de 15% de los casos, por algún factor de memoria materna antimasculina generada por los hermanos biológicos varones mayores criados juntos o aparte [3][4][5][6][7][8][9][10] . Esto debilita la interpretación psicoanalítica de la génesis de la orientación sexual (diamorfofilia según nuestra nomenclatura) 1,2 , ya que el ambiente postnatal influiría menos que el prenatal.…”

“…Esto debilita la interpretación psicoanalítica de la génesis de la orientación sexual (diamorfofilia según nuestra nomenclatura) 1,2 , ya que el ambiente postnatal influiría menos que el prenatal. Este efecto operaría sólo en homosexuales varones por factores inmunitarios [3][4][5][6][7][8][9][10] ; dos propuestas contradictorias con los resultados publicados. Uno de los objetivos de este trabajo es mostrar tal contradicción.…”

Sexual orientation, handedness, sex ratio and fetomaternal tolerance-rejection

“…The probability of being homosexual increases as the number of older brothers increases. Handedness interacts with homosexuality and the SR of siblings; homosexuality, due to FBO, affects non-right-handed men less (Blanchard, 2001(Blanchard, , 2004Blanchard and Bogaert, 1996;Cantor et al, 2002;Bogaert, 2003Bogaert, , 2005Bogaert, , 2006, a relationship explained hypothetically by four prenatal mechanisms Lippa, 2006, 2007;Blanchard, 2008): (1) androgenic factors associated with non-right-handedness could cancel maternal anti-male factors leading to homosexuality; (2) homosexuality and non-right-handedness together may lead to death, neuro-psychic impairment or any condition that confers a low probability of being ascertained; (3) non-right handed fetuses my be insensitive to maternal anti-male factors; (4) mothers of non-right handed fetuses do not produce anti-male factors. The FBO was found also in transsexuals whose sexual identity, rather than their sexual orientation, is altered (Green, 2000).…”