William James, PhD, Honorary Research Fellow of Galton Laboratory at the University College of London, explained in his 2004 paper "The Cause(s) of the Fraternal Birth Order Effect in Male Homosexuality," Journal of Biosocial Science:
"Male homosexuals have a significant excess [more] of older brothers, but not older sisters, as contrasted with male heterosexuals. No such phenomenon was found in the sibships of female homosexuals. These results confirmed a number of earlier studies and the points may reasonably be regarded as established. The phenomenon has been called 'fraternal birth order effect.'"
Does the Fraternal Birth Order Effect Demonstrate a Biological Basis to Sexual Orientation?
Anthony Bogaert, PhD, Associate Professor at Brock University, reported in his July 11, 2006 paper "Biological Versus Nonbiological Older Brothers and Men's Sexual Orientation," published in The Proceedings of the National Academy of Sciences, 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...
If rearing or social factors associated with older male siblings underlies the fraternal birth-order effect, then the number of nonbiological older brothers should predict men's sexual orientation, but they do not... If rearing or social factors underlie the fraternal birth-order effect, the number of biological older brothers with who they were not reared should not predict men's sexual orientation because they should have no impact on the sociosexual environment of their younger brothers. Yet, these brothers do predict men's sexual orientation just as the number of biological older brothers with whom they were reared.
These results support a prenatal [before birth] origin to sexual orientation development in men and indicate that the fraternal birth-order effect is probably the result of a maternal 'memory' for male gestations or births."
Ray Blanchard, PhD, Head of Clinical Sexology at the Centre for Addiction and Mental Health, stated in his June 15, 2004 paper "Quantitative and Theoretical Analyses of the Relation Between Older Brothers and Homosexuality in Men," published in the 2004 Journal of Theoretical Biology:
"The fraternal birth order effect is the finding, based on epidemiological data, that older brothers increase the odds of homosexuality in later-born males... It reflects the progressive immunization of some mothers to male-specific antigens by each succeeding male fetus...
The mother's immune system recognizes these male-specific molecules as foreign and starts producing antibodies to them. Following maternal immunization, maternal anti-male antibodies are available to cross the placental barrier and enter the brain of a male fetus. These antibodies somehow divert the sexual differentiation of the fetal brain from the male-typical pathway, so that the individual will later be attracted to men rather than women."
Louis Gooren, PhD, MD, Chairman of the Genderteam Sex Reassignment Centre at the Free University of Amsterdam, wrote in his Nov. 2006 paper "The Biology of Human Psychosexual Differentiation," published in the 2006 Hormones and Behavior:
"In diverse samples and independent replications, homosexual men are found to have a greater number of older brothers than heterosexual men... [This] certainly does not provide a universal hypothesis for the origins of homosexuality since the majority of homosexual men do have this history and do not fit in this model.
The hypothesis advanced in the above studies is that the late birth order, with more male siblings born earlier, could lead to a progressive immune response of the mother to androgens and/or Y-linked [male specific]... antigens which, by maternal transfer of these immune antibodies to the fetus, could impair brain masculinization of the fetus. However, why this mechanism would selectively impair only certain androgen-dependent processes, such as the brain programming, and not other, like formation of the genitalia, is not explained by this hypothesis, and not even addressed by the proponents. Nor does this theory explain why the majority of boys late in birth order do not become homosexual, even if the elder brother is homosexual...
The biological explanation advanced for the fraternal birth order hypothesis lacks any experimental support."
Neil Whitehead, PhD, Scientific Research Consultant, explained in his Feb. 22, 2007 paper "An Antiboy Antibody? Re-Examination of the Maternal Immune Hypothesis," published in the Journal of Biosocial Science:
"The maternal immune hypothesis (MIH) argues same sex attraction (SSA) results from maternal immune attack on fetal male-specific brain structures and involves the previous biological influence of elder brothers... from the Fraternal Birth Order (FBO) effect...
An attack on 'maleness' should particularly affect development of male genitalia in any fetus which is later SSA-prone. The opposite has been found. From the data gathered by Kinsey, penile lengths were statistically 0.8 cm longer for males with SSA than males with OSA [opposite sex attraction]... Similarly, male puberty should be delayed if there were fetal genital attack, but the very large Add Health longitudinal adolescent study showed no difference in age of puberty between those with SSA and those with OSA...