I had the great pleasure of speaking with Robert Sapolsky — one of the world’s leading neuroscientists and primatologists. A professor at Stanford University, Sapolsky has spent decades studying stress, primate behavior, and the biology of human nature. His university lectures have been viewed by millions of people around the world, helping a global audience better understand why we behave the way we do.

In this conversation, we explore some of the biggest questions about what makes us human. We begin with depression, discussing why it is a serious biological disease rather than simply an extreme form of sadness, and how scientific understanding of depression has evolved over the past two decades. Sapolsky explains the roles of genetics, early-life trauma, epigenetics, and rumination in shaping one of the world’s most devastating mental illnesses.

We then turn to stress, one of the defining themes of Sapolsky’s scientific career. Drawing on more than three decades of studying wild baboons in East Africa, he explains how social support protects us against stress-related disease and why humans and baboons respond to psychological stress in remarkably similar ways.

Finally, we explore what makes humans unlike any other species. Sapolsky reflects on our extraordinary capacity for both compassion and cruelty, the origins of “us versus them” thinking, the unique role of dopamine in motivation and aspiration, and why understanding the biology of human behavior may ultimately help us build a more compassionate society.

Robert Sapolsky is the author of several acclaimed books, including Behave, Why Zebras Don’t Get Ulcers, A Primate’s Memoir, and Determined — his bestselling book exploring the question of free will and what its absence might mean for how we understand ourselves and others.