In long-term relationships, we often expect our beloved to be both best friend and erotic partner. But as Esther Perel argues, good and committed sex draws on two conflicting needs: our need for security and our need for surprise. So how do you sustain desire? With wit and eloquence, Perel lets us in on the mystery of erotic intelligence. Esther Perel speaks on TED about Mating in Captivity.