Another H-Y antigen hypothesis

Teresa Binstock binstoct at
Fri Apr 25 16:00:58 EST 1997

Two new articles:

Blanchard, R., & Klassen, P. (1997). H-Y antigen and homosexuality in men.
Journal of Theoretical Biology, 185, 373-378.

Abstract: In men, sexual orientation correlates with the number of older
brothers, each additional older brother increasing the odds of homosexuality
by approximately 33%.  It is hypothesized that this fraternal birth order
effect reflects the progressive immunization of some mothers to Y-linked
minor histocompatibility antigens (H-Y antigen) by each succeeding male
fetus, and the concomitantly increasing effects of H-Y antibodies on the
sexual differentiation of the brain in each succeeding male fetus.  This
hypothesis is consistent with a variety of evidence, including the apparent
irrelevance of older sisters to the sexual orientation of later-born males,
the probable involvement of H-Y antigen in the development of sex-typical
traits, and the detrimental effects of immunization of female mice to H-Y
antigen on the reproductive performance of subsequent male offspring.


Blanchard, R., & Bogaert, A. F. (1997). Additive effects of older brothers
and homosexual brothers in the prediction of marriage and cohabitation.
Behavior Genetics, 27, 45-54.

Abstract: Research has shown that male homosexuality tends to cluster in
families and that homosexual males have, on average, a greater number of
older brothers than do heterosexual males.  This study investigated whether
the former, between-families effect and the latter, within-families effect
are additive.  The subjects were 717 full siblings over age 40 reported by
343 heterosexual and homosexual male probands examined in Southern Ontario
in 1994-1995.  The sibling's history of legal marriage or cohabitation in a
heterosexual relationship was taken as a proxy variable for sexual
orientation.  There were no significant findings for the female siblings.
As expected, the never-married male siblings were more likely to come from
the sibships of the homosexual probands, and they had a greater average
number of older brothers.  A bootstrapped logistic regression analysis
showed that an additive model best explained the male siblings' data.  The
results suggest that the familial aggregation of male homosexuality can not
be explained by the birth order effect, and that older brothers and family
membership reflect separate influences on sexual orientation or
sexual-orientation-correlated behavior.


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