Psychologist Jillian Jordan, who led the Yale experiment, said she wasn’t trying to suggest that people were faking outrage for the purpose of looking good. She believes people genuinely feel the outrage. The point was to explain the urge to share it so ostentatiously.

In real-world cases, most people unconsciously tally costs and benefits, said Harvard psychologist Max Krasnow. There is a cost to outrage, in terms of social risk. The cost shrinks when there are more and more people expressing it in solidarity. If you’re the only person lobbing yogurt at the Icelandic Parliament, you might well get arrested. But if you’re part of a teeming mob, your collective display of outrage can lead to the ousting of the prime minister.

Why do some incidents provoke almost universal outrage and others set off only those in certain age groups or of particular political leanings? One of the most universal sources of outrage is stealing or hoarding resources, said psychologist Eric Pederson. The theory is that this is ingrained in humans because our ancestors’ foraging cultures survived by sharing; if Joe helped himself to what others hunted and gathered, but then did not share his good fortune when he found berries or killed a wildebeest, he’d get in deep trouble.