Sexual orientation, fraternal birth order, and the maternal immune hypothesis: a review.
Level 5 - mechanism / opinion, no new human data
Narrative review presenting mechanistic reasoning and theory without systematic search methodology
PubMed 21315103 · doi:10.1016/j.yfrne.2011.02.004
What was done
Narrative review synthesizing evidence on the fraternal birth order effect in male sexual orientation and evaluating the maternal immune hypothesis as the underlying biological mechanism based on existing literature.
What was found
The abstract reports no quantitative values or statistical effect sizes. It notes that the fraternal birth order effect is replicated and prenatal in origin, and describes evidence that maternal immune responses against Y-linked proteins (such as H-Y antigen/SMCY) increase in likelihood with each male pregnancy, potentially targeting male fetal brain cells to alter sexual differentiation.
Why it matters
The review provides a theoretical and biological framework proposing maternal isoimmunization against Y-linked proteins as the driver of the fraternal birth order effect.
Limits
This is a narrative review rather than a systematic review or meta-analysis. The abstract notes that direct empirical support for the exact prenatal mechanism is lacking and notes methodological challenges in testing the hypothesis.
Cited by
- supports A baby boy with no older brothers has about a 2% baseline probability of being gay, and each older brother increases the odds by roughly 33% (to approximately 2.6% for one older brother and 3.5% for two older brothers).
- context Each male pregnancy leaves a biological trace in the mother that biases the likelihood of subsequent male offspring being homosexual.
- supports The fraternal birth order effect on male sexual orientation was first identified and reported by Ray Blanchard.