Are human beings innately violent? And when scientists give us an answer to this question, should we believe them? Killer Instinct is a history of debates about human nature. My aim is not to settle these debates once and for all but rather to examine claims for a “killer instinct” as just that—claims that are created, packaged, and sold to an audience (that either buys or rejects them). I aim to understand the process by which such claims to knowledge are created: what strategies authors use to make their claims persuasive; how they are received; and what happens when they are disputed. My hope is to make twenty-first century readers more savvy and skeptical consumers of scientists’ efforts to tell us why we are the way we are.My story begins in the late 1940s, when scientists who studied animal behavior began presenting themselves to popular audiences as authorities on human nature. Konrad Lorenz, an Austrian ethologist and specialist in bird behavior, made a name for himself by arguing that humans had an instinct for aggression. Lorenz was soon joined by Robert Ardrey, an American science writer convinced by South African fossil finds of the evidence for ancestral human violence. Both authors argued that the human aggression instinct must be acknowledged if it was to be controlled.But even while their view was developing, a rival hypothesis arose to challenge it. This rival claim, brainchild of the American anthropologist Ashley Montagu, held that humans were loving and cooperative by nature. Montagu drew on evidence from cell biology, primatology, and cultural anthropology and gathered his own scientific allies. He derisively dubbed Lorenz and Ardrey “aggressionists” and became their most outspoken critic.Between these two opposing views of human nature, a fierce and protracted debate arose. In the debate, Lorenz and Ardrey took a consistent tack against Montagu. Instead of treating his view as an alternative conception of human nature, they argued that he denied the existence of human nature altogether. Montagu was an “extreme environmentalist,” they said, who held that culture and upbringing shaped human behavior and nothing inherent at all. The critique was a caricature, but it stuck. The debate, rather than a conflict between competing visions of human nature, became a clash of extremes: nature versus nurture.This framing persisted in later iterations of the debate. In 1975, the American entomologist E.O. Wilson proposed “sociobiology” as a new way to understand human nature, as a product of genes selected over eons by environmental pressures. Yet when sociobiology became enmeshed in debate, Wilson took the same tack against his critics as Lorenz and Ardrey had against Montagu. And even Wilson’s successors, the evolutionary psychologists, continued to use the nature-versus-nurture framing well into the twenty-first century.I argue that this framing of nature versus nurture obscures a rich and complex history of twentieth-century ideas about human nature. Even more, it has served as a key strategy that purveyors of the claim for a killer instinct have used to bring that claim to public attention.