We all know what infidelity is, but a universal definition is difficult to carve out—especially in the digital age. Is watching porn cheating, or is it only cheating if the person on the other side of the screen is live? Each scenario is subjective, but psychotherapist Esther Perel crystalizes the three elements that lie at the heart of all cheating: secrecy, sexual alchemy, and emotion—even if the person don't think so. Cheating is typically interpreted as a symptom of a bad relationship or of something lacking in a partner, however one of the biggest revelations for Perel in researching her latest book, The State of Affairs, was that happy people also stray. Even people in satisfying relationships find themselves crossing the line they never thought they would. So what gives? "They often stray not because they want to find another person but because they want to reconnect with a different version of themselves," she says. "It isn’t so much that they want to leave the person that they are with as much as sometimes they want to leave the person that they have themselves become." Esther Perel is the author of The State of Affairs: Rethinking Infidelity. See more at estherperel.com.