From suicide bombings to mass shootings, the nightly news can make it seem like we live in the most violent of times. In barely a century, humanity has seen two world wars and the slaughter of millions by pitiless tyrants, not to mention civil wars, insurgencies, and terrorist attacks. But psychologist Steven Pinker argues that physical violence has been in decline for centuries, and we may be living in one of the most peaceful eras in human history. In this two-hour special, Nova examines Pinker's thesis and takes viewers on a journey through history and the human mind to explore what triggers violence and how it may have decreased over time. Taking clues from a Kenyan archaeology site, modern laboratory experiments, and even literature, researchers trace the social and neurobiological roots of human violence. They look at how forces like income equality and personal contact may curb violence 'interrupters' treat violence like a contagious disease, Nova examines evidence-based approaches to making the world more peaceful