Chapter One A Nation of Migrants The last thing I ever wanted to do was move. Most of what I needed lay within an easy walk of our Cambridgeport apartment. In the mornings, I took my children to preschool, stopping to run errands at the local shops on the way home. From our back porch, I could see a city park, with a playground and basketball hoops and a broad grassy field. Our neighbors who had been strangers when we moved in had since become our friends; I could pop downstairs for a couple of eggs when I ran short and expect a knock if they needed a cup of sugar. My office at Harvard was just under a mile down the street. But the apartment had only two bedrooms. Rent was costing us a third of our income each month, and it kept going up.

Our daughter and son wouldn''t want to share a room forever, and an apartment with a third bedroom in Cambridge was well beyond our reach. Every year, more friends in our position gave up and left. They moved out to the suburbs or to cheaper cities in other parts of the country--not where they wanted to be, but where they could afford to live. One family in our building had come back to the States with their daughter after years of humanitarian work abroad. When I learned they were expecting twins, I offered my congratulations and was met with a despondent sigh. They''d discovered what a larger apartment in the Boston area would cost--much less a freestanding house--and decided that the only rational thing for them to do was return to Africa. If theirs was an extreme case, it represented a common problem. Every year, fewer Americans can afford to live where they want to.

The crisis shows up in lengthening commutes; the average commuter now travels almost thirty minutes each way to their job, and one in ten spends more than an hour. It shows up in soaring home prices, pushed skyward by the desperation of buyers--up by more than 60 percent over the past decade, to an average of about half a million dollars. It shows up in painfully high rents; half of all renters now spend more than 30 percent of their income on housing, the threshold for being "rent burdened," and a quarter spend more than 50 percent. And it shows up in the record number of Americans unable to afford those rents and forced into homelessness. In fact, it shows up so pervasively that I hardly needed studies and statistics to see it. I thought about it every time I walked past a glitzy new condo for sale across the street from a homeless man sleeping on a sidewalk. I thought about it while sitting in traffic alongside thousands of other drivers who had infinitely better ways to spend their time. I thought about it when I encountered a school in an expensive neighborhood that had been converted to some other use, because so many families had been priced out of the area that Cambridge had lost two-thirds of its children.

I thought about it as I watched anti-gentrification protests, led by neighbors concerned that they, too, would have to leave the city they loved. And I thought about it each month as I wrote a check to cover the rent. Some Americans have become so accustomed to the places with the greatest opportunities being effectively reserved for the rich that it somehow seems natural that they should be. In fact, it represents a recent and profound inversion. For centuries, Americans at the bottom of the economic ladder moved toward such places, not away from them, searching for a foothold on the first rung of that ladder, looking for the chance to climb. Entrepreneurs raced to erect housing to hold them. Cambridge had long been a magnet for such people. The city had hardly run out of space; it hadn''t even returned to the postwar peak of its population.

And yet instead of drawing people in, Cambridge was now driving them out. And it wasn''t alone. Throughout the country, the places where ordinary Americans were likeliest to find better jobs, earn higher incomes, and give their children better lives were increasingly priced beyond their grasp. Families were stressed. Cities were struggling. And the country was growing ever more unequal. What happened? As a historian, I suspected the answers lay in the past, and I started right at home, with my own apartment. Digging through property records, census manuscripts, insurance maps, municipal directories, and old newspapers to piece together its story, I could tell, at a glance, that it hadn''t always been so pricey.

My century-old building was a three-decker--one of the iconic flat-roofed wooden structures, with apartments stacked like layer cakes, built to house the workers of New England''s industrial cities. When they were constructed, far from being seen as luxurious, they were regarded as a sure sign of a neighborhood''s decline. "Foreigners are coming in increasing numbers," the Massachusetts Civic League warned in 1911, bringing with them the three-decker, "which, besides being objectionable on other grounds, is a flimsy fire-trap and a menace to human life." Built in 1901 by Joseph Doherty, a local real estate man who saw opportunity in Cambridgeport''s rapid growth, our menacing building was wider than most, with a central staircase, two units on each floor, and mild delusions of grandeur. If the building showed its age, it also showed a certain pride of craftsmanship. Moldings ran along the ceilings, sliding doors separated the front parlor from the bedroom, and--a delightful absurdity--a butler''s pantry with drawers and shelves opened off the dining room. Doherty had bought the lot and the handsome ten-room, single-family home that occupied it. He tore the house down, replacing it with a pair of double three-deckers and making room for twelve families in a space that had previously accommodated only one.

To the west lay Old Cambridge, home to Harvard and stately mansions on sedate tree-lined streets. In the opposite direction lay East Cambridge, a gritty industrial area filled with factories and recent immigrants. Cambridgeport was a neighborhood in between, in every way--neither an industrial slum nor an elegant suburb, but something new in the world. A settlement house worker named Albert Kennedy studied the neighborhood and watched as the children of immigrants moved in and then moved on to greater prosperity. Deciding he needed a novel term to capture its distinctive role, he dubbed it a "zone of emergence." Doherty had made a clever investment. He had no trouble renting the units in his building, erected opposite a large Catholic church, to other second-generation Irish Americans. The butler''s pantries and other small flourishes he''d added to his buildings appealed to his tenants'' aspirations.

Their parents might have sweated in the factories, but their children were now graduating from high school. Thomas Sweeney, for example, was a self-employed plumber; his daughter Margaret put her education to work as a government stenographer. My building''s six apartments also housed a telephone operator, postal letter carrier, car salesman, store clerk, and gym instructor. Half a century later, the building still housed a similar mix of residents, although as Irish Americans ascended the economic ladder, other second-generation immigrants had arrived behind them. Our own apartment was rented by Jessie White, whose father emigrated from Canada, and her two adult daughters. When Jessie''s husband, a brass finisher, died in 1940, she took in a shipping clerk as a boarder to help make ends meet. One daughter operated a boxing machine at a factory; the other, who had finished high school, found an office job at an oil company. And yet, by the time we moved in half a century after that, something had gone wrong.

Instead of a new generation of blue-collar families, the building held a very different mix of tenants: graduate students, doctors, architects, engineers. The neighborhood was no longer a zone of emergence; only people who had already emerged could possibly afford to move in. And it wasn''t just changing in our building, or on our block. Prices throughout the city had skyrocketed. And in fact, throughout the country, in the areas where the economy was growing fastest and jobs paid the most, demand for housing far outstripped the supply. Today, the problem I''m describing is generally referred to as our affordable housing crisis. But that''s not quite right. If affordability were the underlying problem, then moving to places like Flint, Michigan--where housing is so cheap it can''t be given away--would be the solution.

Americans, though, are not beating a path to Flint, a city of few jobs and bleak prospects. What we actually face is a mobility crisis. The distinction matters. Americans used to be able to choose where to live, but moving toward opportunity is now, largely, a privilege of the economic elite. The notion that people should be able to choose their own communities--instead of being stuck wherever they happen to be born--is America''s most profound contribution to the world. Many of the cherished features of our society trace, in one way or another, back to this innovation; many of our country''s most glaring injustices result from the ways in which this freedom has been denied to those who needed it most. The fact that it is now endangered is not just a problem for housing markets; it''s a lethal threat to the entire American project. A decade ago in Cambridge, as I began researching this book, I didn''t yet understand that.

I knew only that the city where I was born no longer had space for my family--that we were going to have to move, whether we wanted to or not. But that, as it happens, is something Americans have been doing from the very beginning. America is a nation of migrants. No society has ever been remotely so mobile as Ame.