We've all seen someone get testy on a flight, whether it's muttering under their breath or triggering a full-blown brawl that forces the plane to land. (Yup, that happens.)

We usually conclude that travel just makes people act scummy, and that is that. But new research in the Proceedings of the National Academy of Sciences suggests there may be another reason people get so riled up: Those first-class seats.

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Air rage incidents are nearly four times likelier to happen on planes arranged with first-class cabins than planes without them, the Harvard Gazette reports. Researchers analyzed data on in-air spats from the last several years to conclude that economically segregated seating is a strong predictor of such incidents.

The researchers say that passing through a first-class cabin makes other passengers keenly aware of what they don't have, and that makes them angry. First, they're met with the "situational inequality" of having to wait longer to board the plane. Then, there's the "physical inequality" of sitting in a second-rate seat.

As a result, 84 percent of air rage incidents happen in economy class.

That doesn't mean those in the lap of luxury get a pass -- on the flip side, people in first class tend to have higher expectations for the flight experience based on their status, which can prompt a lot of anger if things don't measure up, Harvard sociologist Michael I. Norton told the Gazette.