What makes two people click? What does it really mean to say, “we have chemistry”? The Atlantic's Olga Khazan talks to biological anthropologist Helen Fisher about the four styles of thought and behavior that Fisher has identified through brain scans that help explain the biological underpinnings of romantic love, love addiction, adultery, and divorce. Based on data collected from 35,000 single Americans, Fisher explains modern courtship, why a trend she calls “slow love” makes her optimistic about relationships in the digital age, and how to use brain chemistry to keep love alive.