In 2009, a defence lawyer in Trieste, Italy, successfully argued that his client's possession of the so-called "warrior gene", a variant of the MAOA gene linked to aggressive behaviour, should reduce a murder sentence by a full year. The argument worked. The science, it turns out, did not hold up nearly as well.

Researchers established a connection between the MAOA gene variant and violent behaviour after the 1990s, which the media subsequently reported. But Aysu Okbay, assistant professor of psychiatry and complex trait genetics at........

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