The decade-long, vituperative debate about the aggression instinct is the subject of chapter 5. The earlier chapters, 1 through 4, provide the lead-up, introducing the characters and the stakes on both sides of the debate. The later chapters, 6 and 7 present the denouement, describing what happened after the aggression debate reached its final stalemate and sociobiology entered the fray. The storyline is laid out in the Introduction, so a just-browsing reader might want to begin there to get a sense of the book’s trajectory. Those who want to cut to the main event should then turn to chapter 5.Because the antagonists in the aggression debate were such vivid and colorful characters, the first half of the book introduces them and their worldviews. Who these people were, and what authorial personae they cultivated, mattered to the outcome of the debate. They really were characters, and I hope readers share my fascination with them. Chapter 1 introduces Konrad Lorenz, animal lover and sometime Nazi party member, who shared his home outside Vienna with wild animals, especially wild birds. Chapter 2 turns to Anthony Storr, an English psychiatrist who thought that acknowledging the aggression instinct, especially in men, was the key to healing mental illness. Chapter 3 brings on Robert Ardrey, Broadway playwright and Hollywood scriptwriter, who made a mid-life career change toward science writing when he visited Johannesburg, South Africa and saw his first pre-human fossil skull.In chapter 4 I turn to Ashley Montagu, the English-born American anthropologist who hid his Jewishness under an aristocratic-sounding pseudonym. Montagu worked for UNESCO after World War II, as a critic of the concept of race, then ran afoul of the McCarthyite administrators at Rutgers University, where he was teaching, and was forced out of academia. As a bestselling author and regular on the TV talk show circuit, Montagu became the aggressionists’ main opponent. He gathered a group of allies, among them the Russian-born American sociologist Pitirim Sorokin, who set up a research center at Harvard in the 1950s to study love and altruism, and the American humanistic psychologist Abraham Maslow. Their view of human nature couldn’t have been more different from that of the aggressionists. Readers curious about those branches of the human sciences that study pro-social behavior—cooperation, altruism, and love—might be interested in chapter 4.By viewing sociobiology as the phoenix that rose from the ashes of the aggression debate, and by linking the debate over sociobiology with its predecessor over aggression, I offer a new interpretation of E.O. Wilson, the science he invented, and the controversy it sparked. Readers interested in Wilson and sociobiology might turn to chapters 6 and 7. Chapter 7 argues that Wilson’s key critics were a group of feminist scientists led by the biologist Ruth Hubbard. This group and their critique of sociobiology got much less attention (both from Wilson and from historians) than Wilson’s male critics, and by including them I shed new light on the sociobiology debate. Readers interested in women and feminism in science might especially enjoy this chapter.Since E. O. Wilson’s death in 2021, historians and biographers have been reassessing his legacy. My book contributes to that effort. The criticisms of sociobiology made by feminist biologists in the 1970s and 80s, especially by Ruth Hubbard and her colleagues, were never adequately answered in their day. Hubbard emphasized the variability of female primates’ behavior—as well as that of women across cultures—to dispute any notion of a universal, defining “essence” of femaleness. She also pointed out that the dominance of elite white men in the sciences led to scientific knowledge biased against women and people of color; sociobiology, she said, was a case in point. Wilson laughed off Hubbard’s critiques or otherwise tried to evade them. These criticisms, however, have lost none of their sting and they deserve a hearing. We must make sure that our own sciences of human nature are not susceptible to them.My book is also an attempt to historicize human nature. Despite its pretense of transhistorical universality, the concept of human nature is time-bound and context-dependent, shaped by the historical circumstances in which it takes root. Both the aggressionists and Montagu were products of a war-torn world hungry for human unity. Both were seeking, in the concept of a biological human nature, a basis for human freedom and human rights. By the 1970s, however, partly as a result of the aggression debate, the concept of human nature lost this earlier meaning and instead became freighted with connotations of racism, sexism, and genetic determinism—connotations that persist in the evolutionary psychology popular in our own day. My book explains how this shift happened. I don’t think we can or should go back to the earlier meanings; rather, I aim to show readers that they existed. My larger point is that the concept of human nature has changed over time; it once looked different from the way it does now; and it could change again.Finally, I hope to encourage readers to take all claims about human nature—killer instinct or otherwise—with a grain of salt. Popular media in the United States is replete with the casual anthropomorphizing of animals (from rape in ducks, to chimpanzee politics, to slavery in ants) and zoomorphizing of humans (applying evolutionary logic to explain and justify human behavior). We too often hear the excuse, “it’s in our genes.” Lorenz and Ardrey were not the first to make such cross-species comparisons—both considered themselves heirs to Darwin—but they did bring such analogies an unprecedented level of public notice. Popular evolutionary psychology and popular primatology in the present day follow in the tradition championed by these earlier scientists. Such analogizing, however appealing, is problematic. It is often shot through with pernicious stereotypes and brings us such dangerous claims as the killer instinct. We ought to be on our guard against it.