Perhaps the oldest, most pernicious form of human bias is that of men toward women. It often started at the moment of birth. In ancient Athens, at a public ceremony called the amphidromia, fathers would inspect a newborn and decide whether it would be part of the family, or be cast away. One often socially acceptable reason for abandoning the baby: It was a girl.

Female infanticide has been distressingly common in many societies — and its practice is not just ancient history. In 1990, the Nobel Prize-winning economist Amartya Sen looked at birth ratios in Asia, North Africa, and China and calculated that more than 100 million women were essentially “missing” — meaning that, based on the normal ratio of boys to girls at birth and the longevity of both genders, there was a huge missing number of girls who should have been born, but weren’t.

Sen’s estimate came before the truly widespread adoption of ultrasound tests that could determine the sex of a fetus in utero — which actually made the problem worse, leading to a wave of sex-selective abortions. These were especially common in countries like India and China; the latter’s one-child policy and old biases made families desperate for their one child to be a boy. The Economist has estimated that since 1980 alone, there have been approximately 50 million fewer girls born worldwide than would naturally be expected, which almost certainly means that roughly that nearly all of those girls........

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