Homosexuality | Brothers in arms | Economist.com: "Like many of the best pieces of research, this one raises questions, as well as answering them. One is, how does the mother's body keep count of how many sons she has conceived? A second is, how does that change the environment in the womb? A third is, how does that change affect sexual orientation? And a fourth is, is this an accidental effect, or has it evolved for some reason?
To these questions, Dr Bogaert has no answers, though in some cases he has his suspicions. He speculates that, for reasons as yet unknown, a mother's immune system takes note of the number of male offspring and that each succeeding male fetus is subjected to increased levels of antibodies. These somehow affect its development. Clearly, something strange is going on, because things other than sexual orientation are also affected by birth order. Boys with elder brothers are also likely to have larger-than-normal placentas while in the womb. And despite that apparent nutritional advantage (for a larger placenta should be able to draw more food from the mother's bloodstream), they are also likely to have lower birth-weights than would otherwise be expected.
Dr Blanchard, meanwhile, calculates that about one gay man in seven can chalk his orientation up to having elder brothers. But to the question of whether there is some evolutionary advantage for a mother who has many sons to include a gay one among them, neither he nor Dr Bogaert has an answer.
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