Fraternal Birth Order and Birth Weight in Probably Prehomosexual Feminine Boys

Blanchard, Ray; Zucker, Kenneth J.; Cavacas, Ana; Allin, Sara; Bradley, Susan J.; Schachter, Debbie

doi:10.1006/hbeh.2002.1765

Cited by 59 publications

(46 citation statements)

References 47 publications

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“…In contrast, homosexual and heterosexual males with no older siblings, or with older sisters only, do not differ in birth weight. Similar results were obtained by Blanchard et al (2002). The fact that homosexual males with older brothers, and only homosexual males with older brothers, show reduced birth weights sug-gests that whatever is happening to link fraternal birth order and homosexuality is happening in utero.…”

supporting

confidence: 85%

Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples

Blanchard

Bogaert

2004

American J Hum Biol

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Homosexuality in men correlates with an individual's number of older brothers, greater numbers of older brothers being associated with a greater probability of homosexuality. There are reasons to believe that this relationship is causal rather than merely statistical, that is, that older brothers produce the increase in the probability of homosexuality for later-born males. It is possible, under this assumption, to estimate the proportion of homosexual men who can attribute their sexual orientation to their birth order among their brothers (fraternal birth order). This statistic, the population attributable fraction (PAF), was computed on the combined archival data of 2,256 heterosexual and 71 homosexual men examined in survey studies of sexual behavior in the UK and the USA. The PAF was 28.6%, with 95% confidence limits of 14.8% and 48.0%. These limits encompass the PAF of 15.1% previously estimated with a Canadian sample. The results indicate that the proportion of homosexual men whose sexual orientation is attributable to fraternal birth order constitutes a minority, but not a negligible minority, of all homosexual men. The fraternal birth order effect may reflect the progressive immunization of some mothers to Y-linked antigens by each succeeding male fetus, and the concomitantly increasing effects of antimale antibodies on the sexual differentiation of the brain in each succeeding male fetus.

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supporting

confidence: 85%

Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples

Blanchard

Bogaert

2004

American J Hum Biol

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“…However, due to their interest in the possible impact of older brothers, Blanchard and Ellis noted a 169 g lower average birth weight in homosexuals among those 23% of male probands who had one or more older brothers. A subsequent study comprising 250 Canadian prepubertal boys with gender identity disorder (a group considered to be prehomosexual) and 739 control boys without gender identity problems failed to present results for the overall comparison of birth weights in the two groups (Blanchard et al, 2002). However, in a subgroup analysis of those 7% of study participants who had two or more older brothers, the authors noted a 385 g lower average birth weight in prehomosexual boys than in control boys.…”

Section: Discussionmentioning

confidence: 86%

“…Before us, North American researchers have used other study designs to study possible associations between birth weight and homosexuality in women (Blanchard & Ellis, 2001) and men (Blanchard & Ellis, 2001;Blanchard et al, 2002). Blanchard and Ellis found no overall difference in birth weight between 98 homosexual and 2071 heterosexual women or between 162 homosexual and 898 heterosexual men in their study based on questionnaire data from a convenience sample of 3229 mothers to homosexual and heterosexual probands.…”

Section: Discussionmentioning

confidence: 95%

Body Size at Birth and Same-Sex Marriage in Young Adulthood

Frisch

Zdravkovic

2008

Arch Sex Behav

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An unexplained excess of overweight has been reported among lesbians. In contrast, reports suggest that gay men may be, on average, slightly lighter and shorter than heterosexual men. We studied associations between weight, length, and body mass index (BMI) at birth and same-sex marriage in young adulthood among 818,671 Danes. We used linear regression to calculate differences in mean body measures at birth and Poisson regression analysis to calculate confounder-adjusted incidence rate ratios (IRR) of same-sex marriage according to body measures at birth. Overall, 739 persons entered same-sex marriage at age 18-32 years during 5.6 million person-years of follow-up. Birth year-adjusted mean body measures at birth were similar for same-sex married and other women. However, same-sex marriage rates were 65% higher among women of heavy birth weight (IRR = 1.65; 95% CI = 1.18-2.31, for > or =4000 vs. 3000-3499 g, p = .02), and rates were inversely associated with birth length (p (trend) = .04). For same-sex married men, birth year-adjusted mean weight (-72 g, p = .03), length (-0.3 cm, p = .04), and BMI (-0.1 kg/m(2), p = .09) at birth were lower than for other Danish men. Same-sex marriage rates were increased in men of short birth length (IRR = 1.45; 95% CI = 1.01-2.08, for < or =50 vs. 51-52 cm), although not uniformly so (p (trend) = .16). Our population-based findings suggest that overweight in lesbians may be partly rooted in constitutional factors. Novel findings of smaller average body measures at birth in same-sex marrying men need replication. Factors affecting intrauterine growth may somehow influence sexual and partner-related choices in adulthood.

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“…Blanchard and colleagues have demonstrated that homosexual males (and probably 'pre-homosexual' feminine boys) with older brothers have significantly lower birth weights than homosexual males without older brothers and heterosexual males (and non-effeminate boys) with older brothers (Blanchard & Ellis 2001;Blanchard et al 2002). Additionally, a homosexual orientation is most likely to occur among men with older brothers and shorter stature (Bogaert 2003a).…”

Section: Introductionmentioning

confidence: 99%

The association between the fraternal birth order effect in male homosexuality and other markers of human sexual orientation

Rahman

2005

Biol. Lett.

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Later fraternal birth order (FBO) is a wellestablished correlate of homosexuality in human males and may implicate a maternal immunization response in the feminization of male sexuality. This has led to the suggestion that FBO may relate to other markers of male sexual orientation which are robustly sexually dimorphic. If so, among homosexual males the number of older brothers should strongly correlate with traits such as spatial ability and psychological gender, indicative of greater behavioural feminization, compared to heterosexual males. The present study failed to find significant associations between number of older brothers and these traits.

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Fraternal Birth Order and Birth Weight in Probably Prehomosexual Feminine Boys

Cited by 59 publications

References 47 publications

Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples

Proportion of homosexual men who owe their sexual orientation to fraternal birth order: An estimate based on two national probability samples

Body Size at Birth and Same-Sex Marriage in Young Adulthood

The association between the fraternal birth order effect in male homosexuality and other markers of human sexual orientation

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