Conquest, War, Famine, and Death—except for the last, the four horsemen of the Apocalypse have lost their ability to terrify us. They have been reduced, as a Google search will swiftly demonstrate, to metaphors for bad habits in personal relationships: “criticism, contempt, defensiveness, and stonewalling.” But for most of human history, the horsemen’s shadows were never far from the horizon; being overrun by a conquering army, caught up in the lethal chaos of war, or perishing from hunger were real possibilities, as much a part of the human condition as the certainty of death. Only in the eighteenth century did some European thinkers begin to argue that conquest, war, and famine might be overcome, that history need not be, in Hegel’s memorable phrase, “a slaughter bench at which the happiness of peoples, the wisdom of states, and the virtue of individuals have been sacrificed.”

Optimists offered a number of reasons why violence would decline, most prominently the psychological effects of commercial expansion and material prosperity. Trade, Alexis de Tocqueville believed, was “the natural enemy of all violent passions,” a conviction echoed by liberal theorists throughout the nineteenth century. The destructive energies in modern society revealed by the twentieth century’s two great wars seemed to damage irreparably the optimists’ case, which later generations dismissed with the facile irony that often attends the demise of other people’s dreams.

The Better Angels of Our Nature is a monumental attempt to demonstrate that the optimists were right all along. “This book,” Steven Pinker writes in his preface, “is about what may be the most important thing that has ever happened in human history.... Violence has declined over long stretches of time, and today we may be living in the most peaceable era in our species’ existence.” Pinker begins with a chapter on the prevalence of violence in the past, readily apparent when one turns the bloodstained pages of our culture’s foundational texts. He then considers a series of major developments, each one producing a diminution of violence: the rise of states that can control the murderous anarchy in tribal societies; the “civilizing process” produced by growing economic prosperity and political order; the “humanitarian revolution” that eventually extinguished a number of barbaric practices including the widely accepted institution of slavery; the “long peace” that effectively ended major European wars in the second half of the twentieth century; the “new peace” that now seems to be pacifying much of the world; and the “rights revolutions” that have helped to contain violence against ethnic minorities, women, children, and other frequently victimized groups.

Pinker follows the same procedure in each of these chapters: he assembles the best data he can, counts whenever possible, corrects the statistics for changes in population, and then charts the trajectory of violence, both political and interpersonal. Pinker knows that he is arguing against the grain of scholarly consensus and popular assumptions, so he makes his case in great detail and tries to anticipate objections as he goes along. It seems to me that he establishes his claims beyond the shadow of a doubt: in most of the world, most forms of violence have declined, unevenly and perhaps not permanently, but decisively. We are now closer than at any other moment in human history to that condition of perpetual peace about which Enlightenment philosophers could only dream.

While Pinker makes his case for the decline of violence, he is less successful in explaining why it happened. Pinker is not always able to control the immense range and variety of material he has assembled. I suspect that the concept of “violence,” which is sufficiently capacious to include everything from mass murder to schoolyard bullying, is simply not robust enough to contain all this material in a single explanatory frame. As a result, The Better Angels of Our Nature is more compelling and convincing as a collection of separate parts than as a cohesive whole.

Despite the broad range of historical examples and the rough chronological organization of his first seven chapters, Pinker does not really offer an integrated historical explanation for the decline of violence. The rise of the state, the expansion of commerce, the growth of humanitarian sympathies, the decline of major war, the spread of human rights may all point in the same direction, but they are spread across millennia of history, with quite different causes and consequences. It is by no means clear how—or even whether—they fit together.

A second and more important problem with Pinker’s explanatory strategy is the deep and unresolved tension between the book’s seven historical chapters and its last two substantive chapters on psychology, one dealing with the “inner demons” that encourage human violence, the other with those “better angels” that discourage it. I am not competent to judge Pinker’s discussion of the biochemical processes in the brain that may produce or inhibit violent behavior. As Pinker himself admits, our understanding of these matters is still very rudimentary, which makes the implied causality in his reference to the circuitry of the brain that “underlies” our behavior seem something of a stretch.

Pinker devotes most of these two chapters to a number of psychological experiments that seek to measure aggression in controlled (and contrived) laboratory settings. Often these experiments are very cleverly designed, but their connection to actions in the outside world is assumed rather than demonstrated. It is, after all, a long way from the responses of undergraduate volunteers in a university office to the behavior of people confronted with real dangers and opportunities. Nor does Pinker persuade me that the so-called Flynn Effect—based on the philosopher James Flynn’s observation that performance on IQ tests seems to be steadily improving—is a powerful indication of increasing rationality.

The meaning of concepts like intelligence, rationality, and—to cite another of Pinker’s favorites—self-control is the product of a particular historical situation; these are not universal psychological traits that can be measured over time. In twenty-first-century America, for instance, self-control may involve restraining one’s murderous urges, but in other times and places, self-control might mean overcoming a fear of death so that one can act heroically, the kind of self-mastery that sent Hector out to confront Achilles.

Despite these problems, there remains a lot to like and admire in this book. The breadth of Pinker’s reading is truly extraordinarily, from prehistory to the present, criminology to cognitive science, technical philosophy to popular culture. He is usually (but not always) judicious in selecting his scholarly guides and is generous in acknowledging his intellectual debts. While perhaps a bit too lively for some tastes, his writing is accessible, never clogged with jargon or superfluous complexity. At his best, Pinker is able to speak directly to his readers, inviting them to share the excitement of discovery and analysis.

No one will close this book without having encountered many new ideas and being forced to reconsider a number of old ones. I was especially taken with his discussion of “the trajectory of terrorism,” which coolly examines the inflated rhetoric, shaky assumptions, and faulty data on which the United States based the so-called war on terror. Here Pinker provides a model of the sort of well-informed, rigorous analysis that seems to have disappeared from our public discourse. In sum, The Better Angels of Our Nature is a big, serious book, well worth reading, pondering, and quarreling with.