French for “already seen”, déjà vu is “the sort of fleeting, intimate experience that reveals itself more readily to novelists than to researchers. As recently as the 1990’s, social scientists doing population surveys asked about it in the same breath as they inquired about poltergeists and contact with the dead. But new research on memory has opened a promising window on the phenomenon, providing both possible explanations for the sensation and novel ways to create and measure it.”