War is in our genes. Humans are “hard wired” to divide the world into “us” versus “them.” Men have a penchant for rape. We have all encountered claims like these in popular media, where they are often bolstered by scientific authority. I worry that such claims could become self-fulfilling prophecies: believe you are a killer, and you just might start to act like one.All claims about human nature should be treated skeptically. Humanity is unimaginably diverse, and no attempt to define its “essence” will ever capture that diversity. Yet I’m also well aware that the claim for a “beast within” us is a fascinating one, a staple of popular science from Lorenz’s day to our own. I aim to understand how such claims exert their hold over audiences.To do that, I treat claims about human nature as pieces of scientific knowledge that get constructed over time. To understand the process of knowledge-making, I adopt a method from the history of science: study debates. During periods of debate, before consensus forms, it is not yet clear which claims will solidify into fact and which will evaporate into myth or pseudoscience. By studying debates, historians reanimate the alternatives that once existed and help show why one of them prevailed. More than just evidence determines whose view will win out. Who the proponents are, and whose interests they serve, also weigh in the balance. Watching scientific debates unfold puts the process of knowledge-making under a microscope.I examined archival sources, including unpublished letters and manuscripts, in addition to the published record. Using this method, I discovered that there was nothing inevitable about the killer instinct’s rise to prominence in 1960s America. I found that its rise was the result of strategies deliberately used to persuade, of audiences who took up the message and furthered it, of powerful interests that found it appealing, and of a culture obsessed with the problem of violence.Even though the aggression debate took place in the public realm—in popular media and bestselling books—rather than in a laboratory’s private confines, and even though it concerned the “soft” sciences of human nature (not physics or chemistry), I believe that the methods historians have developed for studying debates can apply equally well here. Popular science is still a type of knowledge, one that people regularly encounter on bookstore shelves, in newspapers, and on social media. The process by which it attains the status of knowledge is well worth understanding.My book, then, is the result of my own encounter with the popular science of human nature, my ongoing frustration with some of its claims, and my professional training as a historian of science, which taught me to question how we know what we know.