SPEND SOME TIME thinking about human nature and you’ll inevitably end up grappling with contradictory evidence for what makes us tick. On the one hand, human history is replete with examples of cooperation, problem-solving, and altruism. On the other, humans have been responsible for genocides, slavery and family separation, and indifference or inaction in the face of suffering. Thinkers from Aristotle to Thomas Hobbes, Philippa Foot, and John Gray have attempted to grapple with our essential nature as a species, asking whether we are capable of improvement. In the mid-20th century, the human sciences entered the discussion. As historian Erika Lorraine Milam details in Creatures of Cain: The Hunt for Human Nature in Cold War America, influential researchers believed they could trace humanity’s goodness — or awfulness — to our evolutionary past, our environment, or our genes.

Tracking scientific debates about human nature from the 1940s through the late 1970s, Milam gives particular attention to the rise and fall of the “killer ape” account of human development, which argues that our propensity for violence explains our evolutionary split from other great apes. In essence, murder sets us apart. More broadly, she tracks how the human sciences shaped, and interacted with, American culture at large, by way of middle-school curriculum reform, hyperviolent films, feminism and civil rights, Jane Goodall, and a conservative Congressman who made his Christian faith a centerpiece of his campaigns. Biological essentialism, gender roles, and the causes of racism were all on the table, fiercely debated in the context of theories emerging from primatology, anthropology, biology, and psychology. By focusing on a genre she calls “colloquial science” — books and articles about scientific theories written in accessible language and meant to be read by a broad audience — she is able to show how these debates spilled over from scientific journals or seminar rooms into popular discourse, and the arts and politics, becoming tools for collective self-interpretation.

Those debates were taking place in the aftermath of World War II and against the backdrop of the Cold War. That political context, as Milam makes clear, fueled intense interest in the question of human violence and whether, given the nature of our species, it was even possible to avert a destructive war between nuclear superpowers. Counterintuitively, in the immediate postwar period, many scholars arrived at the optimistic view that humans could indeed improve. As Margaret Mead put it in 1955, “War can become as obsolete as dueling.” This school of thought emphasized the oneness of humanity, its shared origin story. A middle-school curriculum called Man: A Course of Study (MACOS) promulgated this view and, at the height of its popularity, reached over 400,000 students in the United States. Its developers, an anthropologist and a psychologist, hoped early exposure to other cultures would be an antidote to prejudice and racism. To mimic the experience of anthropological fieldwork, they worked with a crew to film the daily life of Netsilik Inuit communities. Although most of the film focused on the cooperation and kindness of the Netsilik, some of it recorded less benign material. Its bloody hunting sequences and references to polygamy and infanticide would later become flashpoints for controversy.

In other words, optimism did not last long — little more than a decade. Milam describes how, by the mid-1960s, three controversial figures became major players — indeed household names — in the debate over human nature: Robert Ardrey, an American playwright and screenwriter turned science popularizer; German sociologist Konrad Lorenz; and an Englishman, zoologist Desmond Morris. All three of them penned successful colloquial science books that shaped the conversation about humanity’s “true” nature. Ardrey’s African Genesis and Territorial Imperative depicted a dark human past in which humanity’s destiny was determined by our species’s invention of weapons. Lorenz’s On Aggression emphasized the struggle between reason and violence. Morris’s The Naked Ape: A Zoologist’s Study of the Human Animal argued that humans evolved to maximize sexual pleasure, an interpretation that delighted the likes of Hugh Hefner. All three thus looked to evolution for an account of human nature, but instead of finding the shared history that had inspired researchers like Mead and the MACOS designers, they found aggression, weapons, and sex as the key traits setting us apart from our evolutionary ancestors. Again, that vision of humanity was particularly compelling — and terrifying — against the backdrop of the Cold War.

The trio’s writings immediately ignited controversy. Feminist anthropologists responded by urging their colleagues to look for more female agency in early hunter-gatherer societies, pointing out that accounts of such societies spilled gallons of ink on weapons while saying little about foraging or agriculture. Civil rights activists and scholars focused on claims that the rise of violence in American cities in the 1960s was related to the natural aggression and territoriality of humans rather than to widespread racial discrimination and violence. In a particularly engaging chapter, Milam examines the work of Colin M. Turnbull and Joseph Towles, anthropologists in an interracial relationship who wrote an influential article in 1968 on “The White Problem in America” for Natural History magazine. The pair struck back against the idea that urban violence was somehow natural or inevitable. Such explanations, they wrote, “avoid the nagging possibility that the violence might stem from something a great deal more serious and basic — the white man himself and his myopia.” In other words, society, not biology, might be the problem.

As liberal academics attacked the killer ape theory from the left, a rising conservative political movement attacked from the right. The restrictive Hays Code of 1930, which governed what movies could and could not depict, fell out of use in the late 1960s, giving birth to a new genre of ultraviolent films portraying masculine aggression as essential to human nature. Sam Peckinpah, director of The Wild Bunch and Straw Dogs, referenced Ardrey, one of the three figures mentioned above, when asked about why he wanted to depict violence on-screen, praising him as a “prophet” who had revealed “the animal nature of man.” Conservative cultural leaders quickly attacked those violent films, arguing that they corrupted American children by creating, rather than reflecting, a culture of violence. The debate also brought attention to what students were being taught in school. John Conlan, a Republican Congressman from Arizona, set his sights on MACOS, arguing that the curriculum depicted humans as violent, cannibalistic, and incestuous, and was itself corrupting of young minds. “MACOS materials are full of references to adultery, cannibalism, killing female babies and old people, trial marriage and wife-swapping, violent murder, and other abhorrent behavior,” he declared, adding that the course had been designed to “alienate [children] from the beliefs and moral values of their parents and local communities.” A contentious series of congressional hearings led to MACOS largely losing its National Science Foundation funding.

The killer ape theory ultimately fell out of fashion in the 1970s not only because it was being assaulted from the left and the right but also because of the rise of two scientific fields. Sociobiology, the first field, emphasized our biological relatedness to (rather than difference from) other species in order to explain human behavior. It privileged human cooperation, or sociality, over violence. Biologist Edward O. Wilson theorized that humans used both cooperation and competition to gain evolutionary advantages, and Richard Dawkins argued that earlier scientists, especially the killer ape proponents, had ignored the importance of individual genetic selection: we evolve as social beings within and against the reality of other members of our species. Intriguingly, however, sociobiologists faced many of the same criticisms that the killer ape proponents had, especially when it came to sexism: their descriptions assigned strict gender roles to sex-at-birth. By the time of the 1978 American Association for the Advancement of Science meeting, sociobiology had become so controversial that a group of activists charged the stage where Wilson was speaking and successfully doused him with ice water. Milam argues that sociobiologists’ popular success, and the resulting popular debates and engagement with their theories, precipitated the decline of colloquial science books. Appeals to a broader audience became associated with self-promotion and unwanted attention from nonscientists.

The second set of findings undermining the killer ape theory came from primatology — specifically, Jane Goodall’s chimpanzee research site in Gombe, Tanzania. Most strikingly, mother-daughter chimpanzees Passion and Pom were observed killing and eating infants born to other chimpanzees. There seemed to be little evolutionary advantage to their murder spree — but, notably, the deaths stopped when Pom bore her own infant. Those findings, Milam writes, “proved particularly crucial in dispelling the killer ape notion that human uniqueness lay in our capacity to murder and make war on fellow members of our species.” Humans, it seemed, were not the only species that might kill one another out of anger, stress, or jealousy.

Today, few anthropologists or biologists think that violence is the key to what makes us human. Whatever separates us from other apes is more complicated than a taste for blood. Milam ends Creatures of Cain on an optimistic note, pointing out the recent proliferation of literature and research on the evolution of cooperative human behavior. She writes that “our long, contingent journey from ape to human no longer provokes fear; it provides hope.” But as I closed her book, I found myself most persuaded by Turnbull and Towles, the anthropologists who urged us to chart our future not by examining our evolutionary hardwiring, but by taking a hard look at institutions we’ve built that privilege some people while hurting others. Our nature may well be grounded in a long evolutionary past, but we don’t need to be more evolved to do better. Rather, we need to build institutions and systems that reflect and support the best parts of what it means to be human.

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Melinda Baldwin is a historian of science based at Physics Today. She is the author of Making “Nature”: The History of a Scientific Journal (2015).