Despite the constant news of violence, from mass shootings to wars, psychologist Steven Pinker believes we may be living in one of the most peaceful periods in human existence. Could it be true that physical violence has been in decline for centuries? And can it be prevented—or is it simply part of human nature? NOVA takes you 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 in modern societies. And in places like Baltimore, where violence "interrupters" treat violence like a contagious disease, NOVA examines evidence-based approaches to making the world more peaceful.