In May 2014, a passenger aboard a flight to Raleigh-Durham airport puked. The sight caused five other people to follow suit, forcing an emergency landing. The phenomenon is called “sympathy vomiting,” and it may be caused by mirror neurons, which help us feel empathy and cause people to imitate the behavior of others. Researchers speculate that they’re why yawns are contagious; why your eyes well up when a friend cries; and why the Jackass franchise makes men afraid to go outside without wearing a protective cup. But an ancient survival instinct is also at work. In our cave-dwelling past, any toxin that one person ingested was likely devoured by an entire group. If someone lost their lunch, it was a good idea for everyone else to upchuck too.