United Airlines has apologized for an incident in which an Asian man was violently removed from a flight on Sunday night, claiming that it was an unfortunate case of overbooking.

Four passengers were ordered to leave the airplane as determined by United’s passenger contract which uses “fare class [and] status of frequent flyer program” to determine priority for passenger denial.

Commentators have pointed out that airline classes, with perks like shorter security lines, larger seats, and even better cabin air quality are visible representations of an ever-growing class divide among society.

For a look at how inequality plays out in the airline industry, we turn to social psychologist Keith Payne, author of the upcoming book The Broken Ladder: How Inequality Affects the Way We Think, Live, and Die, and Edward Hasbrouck, a travel writer and civil liberties activist.