Big events go by unseen while we sweat the smaller stuff; things happen underground while we watch the boulevard parades. Truly underground, sometimes: in 1858, the pundits and politicians in Britain were obsessing over the British government’s takeover of India from the East India Company and the intentions of Napoleon III, yet the really big thing was the construction, with the supervisory genius of the great engineer Joseph Bazalgette, of a sewer system to protect London from its own waste, and so arrest the smelly “miasma” that had come to crisis conditions that year. This underground system, along with its visible embankments, would, both directly and by example, save countless lives in the developed world during the next century—making cholera epidemics, for instance, a thing of the distant past. But it got built in relative invisibility.

In the United States over the past three decades, while people argue about tax cuts and terrorism, the wave of social change that has most altered the shape of American life, as much as the new embankments of the Thames changed life then, has been what the N.Y.U. sociologist Patrick Sharkey calls “the great crime decline.” The term, which seems to have originated with the influential Berkeley criminologist Franklin E. Zimring, refers to the still puzzling disappearance from our big-city streets of violent crime, so long the warping force of American life—driving white flight to the suburbs and fuelling the rise of Richard Nixon and Ronald Reagan, not to mention the career of Martin Scorsese. (“Taxi Driver” is the great poem of New York around the height of high crime, with steam coming out of the hellish manholes and violence recumbent in the back seat.) No one saw it coming, and the still odder thing is that, once it came, no one seemed adequately equipped to praise it.

Sharkey, who came of age in that safer era, intends to be its eulogist. He begins his remarkable new book, “Uneasy Peace: The Great Crime Decline, the Renewal of City Life, and the Next War on Violence” (Norton), in the South Bronx, at a city block near Yankee Stadium, and recalls a time in the nineteen-seventies, whose climax was the fearsome blackout riots of 1977, when even the Stadium was sparsely attended. “In some years, night games drew ten thousand fewer fans than day games,” he writes; many New Yorkers were unwilling to make their way into the Bronx after dark. “Spaces that had been created to support public life, to be enjoyed by all—those that define city life in America’s greatest metropolis—were dominated by the threat of violence.” Now, he says, “the calm of Franz Sigel Park reflected the atmosphere of peace through New York City. In the city where more than 2,000 people used to be murdered each year, 328 were killed in 2014, the lowest tally since the first half of the twentieth century.” (Last year, the tally was still lower.) It wasn’t just New York. Violent crime fell in Atlanta, Dallas, Los Angeles, and Washington, and not by a little but by a lot.

More important, the quality of life changed dramatically, particularly for the most vulnerable. Sharkey, studying the crime decline in six American cities, concludes, “As the degree of violence has fallen, the gap between the neighborhoods of the poor and nonpoor has narrowed.” In Cleveland in the eighties, the level of violence in poor neighborhoods was about seventy per cent higher than in the rest of the city; by 2010, that number had dropped to twenty-four per cent. The reduction of fear allowed much else to blossom: “Subway cars, commuter lines, and buses in U.S. cities filled up, as residents and commuters became more willing to leave their cars behind and travel to and from work together. . . . Fans came back to Yankee Stadium in the Bronx, and just as many began to show up for night games as for day games.” The big city was revived. From Portland, Maine, to Portland, Oregon, the transformation of America’s inner cities from wastelands to self-conscious espresso zones became the comedy of our time.

Yet little trace of this transformation troubles our art, or even much of our public discourse. Our pundits either take the great crime decline for granted or focus on the troubles it has helped create, like high housing prices in San Francisco or Brooklyn. Even when we pay attention to the comedy, we rarely look at the cause. Some of our politicians even pretend it hasn’t happened, with Donald Trump continuing to campaign against crime and carnage where it scarcely exists. (If people really thought that urban crime still flourished, of course, he wouldn’t be able to sell condos with his name on them on the far West Side of Manhattan.) Attorney General Jeff Sessions, meanwhile, feels free to tell the outrageous lie that “for the first time in a long time, Americans can have hope for a safer future.”

This lack of appreciation is partly a question of media attention-deficit disorder: if there is little news value in Dog Bites Man, there is none whatever in Dog Does Not Bite Man. It is part of the neutral unseen background of events, even if there had previously been an epidemic of dog bites. But it’s hard for those who didn’t live through the great crime wave of the sixties, seventies, and eighties to fully understand the scale or the horror of it, or the improbability of its end. Every set of blocks had its detours; a new arrival in New York was told always to carry a ten-dollar bill in case of a mugging. Crime ruled Broadway comedies: Neil Simon’s “The Prisoner of Second Avenue” told the tale of people barricaded in their apartments for fear of muggings. My great-aunt and great-uncle lived on 115th and Riverside Drive; an address they boasted of in 1962 had become a neighborhood they were frightened to have company visit by 1975. For those trapped in true low-income, high-crime communities, these circumstances were even worse, with, as Sharkey shows, catastrophic effects not only on life and limb and property but on the fundamental human capacity for hope. In every way, the crime wave had effects far wider-reaching than its emergency-ward casualties. Liberal urbanists, who had been, perhaps mostly by chance, in power when the crime wave began, were discredited for a generation. The neocons gained credibility on foreign policy because they once seemed right about the Upper West Side.

Sharkey, unlike many of his peers on the left, regards the great decline as an unmediated good, benefitting everyone, and, above all, the poorest and most vulnerable. Sharkey’s book, in fact, illustrates why social science, with all its uncertainties—uncertainties built into a field in which you are studying the actions of several million autonomous agents who can alter their actions at a whim, with several thousand outliers guaranteed in advance to be bizarrely atypical—still really is science. What makes it science is what makes it social: an insistence on paying attention to the facts that other people have gathered even when they conflict with the way you want the world to be; a reluctance to tailor the facts to one’s views, instead of one’s views to the facts.

You might wonder that anyone would dispute the notion that the crime decline is a good thing for everyone, but some do, either sentimentally—what ever became of all the lively crack dealers and Forty-second Street prostitutes?—or sententiously: a “cleaned up” city dismissed as merely sanitized, with the social problems pushed to the periphery. Sharkey, a sympathizer with progressive causes, sees the position in which urban crime is taken to be a kind of political violence—an as yet insufficiently organized program of dissent—for the academic indulgence that it is. The view that violent crime is a kind of instinctive form of political protest is not a new one, or entirely outlandish. We take it for granted, thinking of the poverty-stricken thieves, hanged for stealing handkerchiefs in eighteenth-century London, that the argument of the “The Beggar’s Opera” is not wrong: even when not explicitly political, crime can have an implicit politics. But though these arguments—like the parallel ones about when terrorism becomes patriotism and patriotism terrorism—are easy to make, they are hard to use as helpful guides to the real world.

Sharkey’s own research began with a simple experiment by the neuroscientist David Diamond, of the University of South Florida. Diamond placed a cat outside a cage of rats, and found that rats raised in this condition did worse on rat-friendly cognitive tests—running complicated mazes and the like—than did rats kept away from the sight of cats. You might imagine that rats raised in the presence of a predator learn to be shrewder. But this seems not to be true of rats raised in the presence of a predator whom they can do nothing to avoid or outwit—rats that feel helpless in the face of, so to speak, a cat wave. Brains under stress get frozen.

Sharkey’s subsequent research showed that children respond to the stress of community violence in a similar way. When children take a standardized test shortly after a neighborhood murder, their scores suffer. The price of crime is paid, above all, by the trauma of kids whose parents can’t buy their way out of its presence. “Local violence does not make children less intelligent,” Sharkey says. “Rather, it occupies their minds.” Thinking about a threat leaves you less room to think about anything else. The social cost of street crime, therefore, is far higher than the price of lives lost and bodies maimed; it can maim minds, too. Conversely, Sharkey finds that, in places where violence has declined the most, kids do much better at school, and minority kids lag least. Anyone who says that the decline in crime is a white person’s prerogative and pleasure hasn’t been following the facts.

But what made the crime wave happen and what made it halt? As liberal-minded people, we want the real cause of the crime decline to be nice people doing nice things, with no role for nasty people doing nasty things to those still nastier. And Sharkey does make heroes, persuasively, of many nice people doing nice things to stop crime. He is an enthusiast of the hypothesis that local community organizing was a key factor in the crime drop: “It was hard work by residents, organized into community groups and block clubs, that transformed urban neighborhoods.” He thinks that technology—surveillance cameras, LoJack systems—played a part. But he also finds that incarceration accounted for some of the crime decline, and so did more aggressive policing. “Federal funding paid for tens of thousands of new police officers,” he writes. “The tactics they used were sometimes oppressive and sometimes brutal but were also more effective, focusing resources on the precise locations where crime was most intense.”

Here some ambiguity arises. What’s now called stop-and-frisk policing—in which police aggressively sought out suspected minor criminals on the streets, most of them minority kids—was, he suggests, a kind of schismatic variant of what had originally been called, more benignly, “broken windows” policing. As Sharkey notes, broken-windows policing was based on a theory that was offered without any real evidentiary basis, and published in The Atlantic, not in a peer-reviewed journal. What was easily missed was that the broken-windows tactic, as first articulated by the criminologists James Q. Wilson and George L. Kelling in 1982, was not an appeal to the power of policing. It was an appeal to the power of self-policing. At a time when policing had been reduced, in many American cities, to having wary patrolmen drive around in squad cars, waiting for a radio call telling them that something bad had already happened, the new theory insisted on an aggressive pursuit of petty crime, before it could get to be big crime. If the cops led the way—and this was the crucial idea—the community would follow. Blocks left intact, windows repaired by conscientious landlords, would produce the eyes on the street and the small-scale intense local engagement that had in the past assured the safety of local neighborhoods. South Philadelphia or the Bronx’s Grand Concourse in the nineteen-fifties had been largely benign places not because the police were present but because there were so many engaged passersby that the police didn’t need to be present.

This sane theory of self-policing soon became its own opposite. A new, noxious notion grew. It would be all police, all the time. If enough policemen frisked enough young minority kids, they would find enough weed and weaponry to send them away. Once sent away, the potential criminals—known to be so owing to their possession of weed and weaponry—could not be street criminals, by dint of not being on the street. By the time they were back outside, the window of opportunity for committing crimes would have largely passed, crime, like gymnastics, being an occupation of the young. It was an extraordinarily crude reduction of Wilson and Kelling’s view.

Liberal-minded people do not merely want mass incarceration to be the moral scandal it obviously is. We want it to be a practical scandal as well—it won’t and can’t do any good. But, Sharkey reports, the facts suggest that, for some period and to some measurable degree, it did contribute to the crime decline. It’s just the most expensive, inefficient, and cruel of all ways to combat the crime wave. And the moral horror thereby incurred is intolerable to a liberal democracy that does not want to have millions of men under permanent penal restraint. The social cost of that mass incarceration is as high, in its way, as the crime wave it was meant to hamper. Sharkey’s climactic thesis is that the real challenge for the decades to come is to take advantage of the decline in crime to engineer a parallel decline in incarceration, sending noncareer criminals back to safer streets.