If you were investigating a crime scene, you wouldn’t just accuse the nearest bystander. The real culprit could be miles away.

In 2007, a team of British researchers announced that genetic variants within the FTO gene could predispose people to being fat. On average, people with one set of these variants weighed 1.6 kilograms more than people with none, and those with two sets—including one in six Europeans—weighed 3 kilograms more.

It was an important discovery. By studying twins, scientists had already shown that obesity runs in families, to an extent that can’t be explained by a shared environment. It was clear that some genes can influence how much we weigh (though, obviously, not exclusively*), but no one had identified