It started commonly enough—a group of tourists returning to China from Thailand complained to the staff on their airplane about not being able to sit together. But one member of the group then threatened to blow up the plane after not getting their request, while another later threw a cup of boiling hot water onto a flight attendant after she couldn’t give him change in Chinese currency. Such “air rage” incidents are on the rise, and a new study may reveal why. After analyzing data on “disruptive passenger incidents” from an airline company’s database of millions of domestic and international airline flights, researchers found that flights with a first-class section were nearly four times more likely to have air rage incidents in their economy class, and that these incidents of “belligerent behavior” or “emotional outbursts” became nearly 12 times more likely among first-class passengers and more than twice as likely among economy-class passengers if people were made to board from the front of the plane and walk through the first-class section together. The study, published online today in the Proceedings of the National Academy of Sciences, sought to build on prior research that showed people forced to confront inequality show more antisocial behavior. As first-class sections have become more common and larger since being introduced on airplanes in the late 1970s, they appear to be triggering more air rage incidents among passengers at the back of the plane. The authors argue that airplanes are, in essence, miniature versions of society, where stark differences in class are often a major cause of social unrest.