A groundbreaking work on why people become so divided - and how they can brought back together. All too often, though, these groups become partisan. They start to compete with one another. They become mutually hostile. Why does this happen? They explore the shaping force of our genetic make-up on our fundamental views and the nature of the influences that family, friends and peers exert. They pinpoint the economic and political triggers that tip people from healthy disagreement to dangerous hostility, and the part played by social media in spreading entrenched opinions.