Fraternal birth order and the maternal immune hypothesis of male homosexuality.
Level 5 - mechanism / opinion, no new human data
Narrative review proposing a biological hypothesis with no new empirical human data presented.
PubMed 11534970 · doi:10.1006/hbeh.2001.1681
What was done
The author reviewed evidence for the fraternal birth order effect in male sexual orientation and evaluated the maternal immune hypothesis, which proposes that mothers develop progressive immunization to Y-linked minor histocompatibility (H-Y) antigens across successive male pregnancies.
What was found
The abstract notes that each additional older brother increases a male's odds of homosexuality by approximately 33%. The maternal immune model is supported by indirect observations: older sisters show no effect on later-born male sexual orientation, female mice immunized against H-Y antigens exhibit impaired reproductive performance in subsequent male offspring, heterosexual males with older brothers have lower birth weights than those with older sisters, and homosexual males with older brothers have lower birth weights than heterosexual males with older brothers.
Why it matters
It provides a biological hypothesis—maternal anti-H-Y antibodies crossing the placenta to alter fetal neurodevelopment—to explain the statistical association between older brothers and male sexual orientation.
Limits
The abstract describes a narrative review and hypothesis paper rather than primary experimental data. It lacks specific sample sizes, direct measurements of maternal anti-H-Y antibodies, and direct causal evidence in humans.
Cited by
- supports Each additional older biological brother increases the probability of a male being homosexual by approximately 33%.