Poor and vulnerable communities have benefitted the most from the drop in crime. Illustration by Eiko Ojala Audio: Listen to this story. To hear more feature stories, download the Audm app for your iPhone.

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.