We don't know exactly because it’s difficult to study—you can’t induce déjà vu in most people. But scientists have theories. It may be a memory error: An experience triggers a memory, but the brain can’t retrieve it. When that happens, your brain fails to distinguish the past from the present, leaving you with an odd feeling of familiarity. Another theory holds that two parts of the brain perceive an experience at the same time. If information arrives slightly faster to one part, a person can feel like they’re having the same experience twice. However, a group of scientists from the U.K., France, and Canada think another cause could be anxiety. They recently studied the bizarre case of a 23-year-old man with chronic déjà vu and found in one instance that the more distressed he became by the endless loop of déjà vu experiences, the worse they got. But you’ve probably heard that before.