Ever since Charles Darwin wrote about "evolutionary fitness," this idea has been confused with physical strength and aggression. In fact, what made us evolutionarily fit was a remarkable kind of friendliness, an ability to communicate with others that allowed us to achieve all the cultural and technical marvels in human history. Advancing what they call the "self-domestication theory," Brian Hare and Vanessa Woods shed light on the mysterious leap in human cognition that allowed Homo sapiens to thrive—but also made us the cruelest species on earth.