Ray Blanchard, a researcher at Toronto's Centre for Addiction and Mental Health, was reviewing some data a few years ago when he noticed something odd: gay men seemed to have more older brothers than straight men. Intrigued—and sceptical—he decided to investigate. He recruited 302 gay men and the same number of heterosexual controls and inquired about their families. How many siblings did they have, of what sex, and how had the births been spaced? How old had their parents been when they had had them? Dr Blanchard found that only one detail seemed to predict sexual orientation: the more elder brothers a man had, the more likely he was to be gay. Neither elder sisters nor younger siblings of either sex had any effect, but each additional elder brother increased his chance of being gay by about 33% from the population average of one man in 50.
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