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In the last two decades, dozens of scientific papers have been published on the biological origins of homosexuality - another announcement was made last week. It's becoming scientific orthodoxy. But how does it fit with Darwin's theory of evolution? Macklemore and Ryan Lewis's hit song Same Love, which has become an unofficial anthem of the pro-gay marriage campaign in the US, reflects how many gay people feel about their sexuality. It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition". A minority of gay people disagree, maintaining that sexuality is a social construct, and they have made a conscious, proud choiceto take same-sex partners. But scientific opinion is with Macklemore. Since the early 1990s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal line, and a genetic factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals. But since gay and lesbian people have fewer children than straight people, a problem arises. "This is a paradox from an evolutionary perspective," says Paul Vasey from the University of Lethbridge in Canada. "How can a trait like male homosexuality, which has a genetic component, persist over evolutionary time if the individuals that carry the genes associated with that trait are not reproducing?" | 
