Greg Klee/Globe Staff

Somewhere in the world, every second of every day, people are being beaten, shot, and stabbed. The news is a litany of bombings and political assassinations, deadly riots and gang warfare. The lucky among us merely hear about it. Some days, when the body count is particularly high, it can be hard to stave off the sense that our species is more brutal and more bloodthirsty than at any other point in history.

Steven Pinker used to wince at the carnage like everybody else, and wonder how the human race had managed to lose its way so horribly. Then, in 1989, he stumbled upon something remarkable: a graph in a history book, compiled by a political scientist named Ted Robert Gurr, showing that the homicide rate in England had declined sharply since the 13th century. Pinker was astonished. The rate had fallen in some areas by as much as one hundredfold. Could it be true, he wondered, that humans had actually become less violent with time, as opposed to more? And if so, how had we done it?

Pinker, now a psychology professor at Harvard, was then a rising star at MIT known primarily for his work on how the mind processes language and vision. In the years after his eye-opening encounter with the Gurr graph, his interest in broader questions about human nature and the brain would lead him to write a series of bestselling books, including “How the Mind Works” and “The Blank Slate,” which helped establish him as one of the most recognizable public intellectuals of the past 20 years.

Julia Cumes for The Boston Globe

More than anything, Steven Pinker points to the Enlightenment as increasing the kinds of empathy and tolerance needed for people to live peacefully side by side.

Meanwhile, the counterintuitive notion that human violence was on the decline continued to percolate in Pinker’s mind, and in early 2007, he decided to write a book about it. As he looked deeper into the topic, reading through the work of violence specialists, Pinker found encouraging figures on one front after another: Not only had homicide rates gone down all over the developed world, but so had the amount of war, domestic abuse, rape, slavery, and all kinds of other unspeakable practices. The more Pinker read, the more convinced he became that humans, as a species, had undergone a truly profound transformation.

Just under five years of research and writing later, the result of Pinker’s work is “The Better Angels of Our Nature,” an 800-page volume scheduled for publication in early October, in which he provides nothing less than a comprehensive history of human violence--and attempts to explain its decline over time in terms of his ideas about psychology.

Drawing on everything from archaeological findings and coroners’ reports to court records and official wartime casualty counts, Pinker combs through more than 5,000 years of death and suffering to make the bold and surprising claim that we are living through “the most peaceful era in human history.” Clearly we’ve been doing something right, Pinker argues, and his goal with “Better Angels” is to figure out what that something is--to identify the institutions, values, and intellectual trends that have made us more inclined towards peace and cooperation. The result is a hybrid of history and psychology that ends up being as much an argument about the complexity of human nature as it is the story of a species learning how not to self-destruct.

“[If] reason, science, literacy, democracy, open economies, empowerment of women--all of these things--if you can actually show there are ways in which they’ve made life better,” Pinker said recently, “you remind people that, hey, these things haven’t always been there, and a lot of what we appreciate in life, we should thank these developments for.”

“Better Angels” is a celebration, in other words--an improbable pat on the back from a man who has peered into the darkest parts of the human experience and emerged, against all odds, an emphatic optimist. Even so, Pinker is careful to warn that there is nothing inevitable about the happy trends he has identified. He expects blowback from critics on both sides of the political spectrum who have deep investments in the idea that modernity is fundamentally flawed. And as far as we may have come, he believes there is always going to be a possibility that circumstances will change and our “inner demons”--as opposed to the “better angels” that give the book its title--will once again rear their heads. It’s for that very reason, Pinker says, that we must try to be clear-eyed about the trajectory we are on, and aggressive in our attempts to stay on it.

Trying to measure the amount of violence in the world at any given time sounds at first blush like an impossible task, not least because violence comes in so many different forms, and so often goes unrecorded. “Better Angels” is a book loaded with data, but reading it, one never forgets just how much ingenuity and care is required to extract meaningful information from the existing records. Even in cases where data were systematically collected-- government officials in France and England started keeping track of crime statistics and homicide rates in the 19th century--it still only captures a fraction of what really happened.

Sitting on the couch in the spacious and pristine apartment in the Leather District that he shares with his wife, the novelist and philosopher Rebecca Goldstein, Pinker explained that he had no illusions about the quality of the available records. There are holes everywhere, he said, with wide margins of error clouding many of the statistics. Ultimately, though, he doesn’t think it poses a problem for his thesis. “The point is not to say exactly 700,322 people died in a particular year,” he said, “but to ask whether it’s more or less than the year before.”

Though Pinker is a scientist by training, “Better Angels” does not pretend to be a work of original research, and Pinker is the first to admit that he is no more than an amateur historian. His argument is built almost entirely on the work of other scholars, many of whom have spent years painstakingly collecting data on the prevalence of various kinds of violence in various regions throughout history. By and large, it seems these specialists have welcomed Pinker into their midst over the past several years, inviting him to their conferences and watching with appreciation as he has tried to understand and analyze their findings. “For our field, I have to just confess, it was, ‘Jeez, Steve Pinker’s interested in our work!’ That was our reaction,” said Randolph Roth, a professor at Ohio State University who studies the history of violence in America, and who is the codirector of an international clearinghouse for violence research called the Historical Violence Database. “He’s been very rigorous about this--trying to understand the strengths and weaknesses of our numbers, and asking very good questions about what we do and don’t understand.”

Pinker’s appetite for information is plainly reflected in the sprightly promiscuity with which he jumps from one measure of violence to another in the course of telling his story. Reading “Better Angels,” one quickly gets used to seeing, for instance, a graph on British aristocrats who died violent deaths from 1330 through 1829 just a few pages away from a bit about the practice of cutting people’s noses off in medieval Europe. “He’s putting this all together, and that’s really a major achievement,” said John Mueller, a political scientist at Ohio State who is cited throughout “Better Angels” for his work on the history of war.

By taking such a long view and considering so many different kinds of violence, Pinker hopes to provide a corrective to what he calls the myopia of recent memory. One of his priorities, he said, was to debunk conventional wisdom about the 20th century being the bloodiest of all time--a notion rooted above all in the horrific, mechanized mass killings of World War II and the Holocaust. In absolute terms, Pinker says, the war was undoubtedly the deadliest event in human history. But to see it as representative of modernity is a mistake--in fact, he argues, the war was an upward spike in the middle of a long downward slope, or “the last gasp in a long slide of major war into historical obsolescence.” Since the Middle Ages, Pinker shows, wars between developed nations have grown shorter and less numerous, with the yearly number of conflicts between European states falling from more than three per year in 1400 to under one per year in 2000, and the average number of battle deaths incurred during interstate conflict falling from the 1950s figure of around 70,000 per year to fewer than 10,000 today.

In rendering the long-term trends in violence and trying to explain their origins, Pinker identifies a few key developments in deep history--namely, the transition from anarchic tribalism to state-run society that began 5,000 years ago, and the emergence of “gentle commerce” that made it more profitable to do business with other people than to kill them for their resources. But more than anything, Pinker points to the Enlightenment, and much of “Better Angels” amounts to a psychological argument that the ideas that came out of it--secularism, reason, science, the notion that the rights of the individual should be privileged over that of the group--can reasonably be credited with increasing the kinds of empathy and tolerance needed for people to live peacefully side by side.

It was because of those historical developments, Pinker argues, that humans have been able to escape what he calls the “pacifist’s dilemma,” in which everyone wants to put down their arms and cooperate, but no one is willing to do so unless they can be sure their rivals will as well. As Pinker imagines it, the tension between embracing our neighbors and baring our teeth in self-defense comes down to a struggle between two competing sets of psychological urges: one that includes impulses toward dominance, revenge, sadism, and ideology, and another that allows for empathy, self-control, and reason. It is Pinker’s belief that over the course of our development as a civilization the “better angels” that comprise the second category have won out over the “inner demons” of the first because the social structures we have invented, along with the spread of literacy and cosmopolitanism, have made them increasingly advantageous. The complex economies we participate in, the democracies in which we elect people to govern us--these enable and reinforce our instinct for cooperation, Pinker argues, and check the impulse to hurt one another in order to get what we want.

“We’re all amazed at the tremendous capacity of human beings for violence and nonviolence, for cooperation and aggression,” said Roth. What Pinker has attempted, he added, is to explain how the “same biological unit in one circumstance can be the deadliest species on earth and then flip around the next moment and be the most loving, nurturing, self-sacrificing, altruistic species that’s ever lived.”

As warmly as Pinker seems to have been received by historians of violence since he wandered into their backyard, and as strongly as they agree with him that mankind has grown less violent over time, there is no consensus that his interpretation of the trend is the right one.

Mueller, for example, is not convinced there is a coherent set of underlying causes behind the declines in various kinds of violence. The way he sees it, the world has changed in a more haphazard fashion, through independent ideas that are promoted, with varying success, by “idea entrepreneurs” who are either good or bad--not to mention lucky or unlucky--at popularizing their beliefs. Just because human rights are in vogue today, in other words, there’s no reason to think people won’t be swept away by some other, less enlightened doctrine tomorrow. For James Payne, whose self-published 2004 book “A History of Force” Pinker called a “major influence,” there is simply no way to know why people have become more peaceful--it’s a mystery, according to Payne, and one that cannot be definitively solved because we simply can’t go back and test our theories. “It’s one thing to see the trend,” Payne said, “and the trend is really quite clear the more you look carefully at the historical record--but [as for] what caused it? We’re all a little bit at sea.”

For Pinker, meanwhile, explaining the trend is the whole point of identifying it in the first place. Otherwise, he reasons, we’re giving up on the possibility of learning anything from what has to be one of the most encouraging trend lines in human history. And while Pinker is reluctant to make any policy prescriptions based on his insights on violence, he is predicting that his book will inspire condemnations both from thinkers on the left who lament the corrupting forces of modernity, and from so-called theo-conservatives on the right who long for a time when people were united by religion, tradition, and moral clarity.

Ultimately, Pinker hopes “Better Angels” will serve as a decisive defense of modern life--a testament to the fact that for all the complaints one hears about contemporary depravity and nihilism, and for all the carnage in the headlines, mankind deserves credit for having managed to largely free itself from its violent, beastly instincts.

“I have a hope for a kind of reappreciation for some of the forces of modernity, like reason, like literacy, like science--which have often been blamed for the catastrophes of the 20th century,” said Pinker. He added, “If the numbers suggest that, contrary to appearances, the long-term historical trend is for violence to go down...the entire movement away from tribe, community, tradition, and religion towards secularism, science, rationality, internationalism...has been a good thing.”

This optimistic bigger picture is important to remember as we’re confronted with all the ways in which our world is still troubled and cruel. “If you don’t have a sense of whether things are getting better or worse, you really don’t have a good basis for forming policy,” said Joshua Goldstein, a political scientist at UMass-Amherst and the author of a new book called “Winning the War on War: The Decline of Armed Conflict Worldwide.” “It’s like if you had cancer, but you didn’t know whether it was spreading or going into remission, then you wouldn’t know whether to continue using the treatment you’re using or to try something radical and new. What we’re doing is working.”