Variations in male sexual orientation: The role of maternal responsivity to male-specific proteins

兄弟会出生顺序(FBO)效应--平均而言，同性恋男性比异性恋男性有更多的哥哥--是男性性取向的可靠关联。这种影响很可能在出生之前就出现了，最合理的解释是，在怀孕期间，母亲对一种对男性胎儿发育至关重要的物质产生了免疫反应，这种影响随着每次男性怀孕而变得更有可能，并被认为会导致一些后来出生的男性的大脑发育发生变化。反应的目标可能是胎儿脑细胞表面的蛋白质分子，因此与它们结合的母体抗体会改变它们在典型的性别分化中的作用，导致一些后来出生的男性被男性吸引，而不是女性。

The fraternal birth order (FBO) effect—that, on average, gay men have a higher number of older brothers than heterosexual men—is a reliable correlate of men's sexual orientation. This effect is likely to arise before birth, with the most plausible explanation being that during pregnancy a mother develops an immune response to a substance important in male fetal development, an effect that becomes more likely with each male gestation and is thought to cause an alteration in the brain development of some later born males. The target of the response may be protein molecules on the surface of fetal brain cells so that maternal antibodies binding to them alter their role in typical sexual differentiation, leading some later born males to be attracted to men not women.