Urban areas are defined globally as continuously "built up" land within a labor market. They are typically larger than cities but smaller than metropolitan areas.

LAST month, in response to the news that Detroit's white population is now growing for the first time in decades, with the number of residents surging in particular downtown, a local radio station paused to ask: "Is Detroit big enough for everyone?"

It was an odd question, given that the city's population is less than half the size it was in 1950, with tens of thousands of empty lots and hollow homes attesting to the ample elbow room. If any community in America has space — crannies to tuck new housing, capacity to absorb more ideas and bodies — it is Detroit.

Echoes of a similar suspicion to the contrary, though, are widespread in how we talk about the places where we live. The entire city of San Francisco is "cooked. Done." There's no more room in Silicon Valley, either. Brooklyn is at capacity. Boston, too. The nicest parts of Northwest Washington long ago reached the limit. Chicago's coveted Lincoln Park wants fewer people. Even whole countries now suffer from this condition: Britain just can't take in any of those refugees because the island, at long last, is full.

Built into these arguments is a powerful but slippery contention: It is possible to fill up a place.

We seldom question the premise because it sounds absolute, measurable, even mathematical. I'm sorry, it's not that I don't want you to move here. It's just that we are already full.

But a neighborhood, no less a continent, defies the knowable volume of boxes or bottles or stadiums. We know when a bathtub is full, because the water runs over. Same with a rush-hour bus, when the bodies won't budge. But a city? A city block? How would you even define such a thing?

"Economists reject absolutes like 'full' and 'need,'" says Joe Cortright, one of several urban economists I asked. "It’s always about tradeoffs and choices."

Cities, in particular, are about tradeoffs between hectic streets and vibrant economies, between scarce parking and neighborhoods worth traveling to, between cramped subway cars and the mass of humanity that makes the subway possible.

And so, from an economist's point of view, there is no such thing as a full place. Especially not in America, where our neighborhoods, as urban planning professor Sonia Hirt puts it, are "astonishingly low density" compared to the rest of the industrialized world. Maybe your particular geology can't handle the foundation of a mile-high skyscraper. But, for the most part, we can always make choices to make more room, to build taller and denser, to upgrade schools and rethink roads to let more people in.

That we don't isn't a limitation of physics. It's a matter of politics disguised as physics.

"When people say a place is 'full,' to me it’s shorthand for they’re not willing to even entertain the challenges of what it would mean to redevelop the space," says George McCarthy, the president of the Lincoln Institute of Land Policy.

"One of the things about being 'full' — or saying you’re 'full' — is the conclusion that the quality of life in the place will never be better than it is right now," he goes on. "That’s what people are really saying. They’re saying 'any change you make is going to require a sacrifice of one sort or another that we’re not willing to make.'"

By this logic, the latest person to move to San Francisco, or Portland, or even Detroit is always, miraculously, the last one to squeeze in before the gates must slam shut.

Says McCarthy: “This is the American mantra: I’m here, all development can stop.”

A deep misunderstanding of cities

Everything we know about cities suggests that, in fact, quality of life doesn't go down as more people crowd in — the opposite happens. Denser populations support not just more amenities (museums, baseball teams, restaurants) but more diverse amenities (Korean and Ethiopian and Peruvian joints!).

This is why so many people want to live in San Francisco in the first place, and why tourists like to visit Manhattan. The things there worth experiencing are there precisely because so many other people are, too.

The tight confines of cities make us more energy-efficient, too, as economist Ed Glaeser has argued, because 20 households in one apartment building devour fewer resources than 20 households in single-family homes.

Put more and more workers in one place, meanwhile, and you also get buzzing hubs like New York's Garment District or Boston's biotech corridor, where people working on the same problems bump into each other and share ideas and suppliers and become more productive. Put more people in a city, and the economy grows. It's the opposite of diminishing returns. The environmental costs, per person, can actually improve. In drought-stricken California, some of the lowest water consumption per capita is in San Francisco.

"To claim that an area can 'fill up' is to implicitly assume that there’s an ecological footprint relationship, like a law of physics, such that every extra person uses the same amount of resources," says Matthew Kahn, a visiting professor of economics at USC. "But the whole point of economics is that we can always substitute inputs. We can build up, build down. There’s always substitution possibilities such that you can achieve more with less."

Of course, there are costs along the way — and tricky ones — to adding more people. They may bring congestion and pollution. It becomes harder to park. Schools fill up. But we have ways of dealing with those downsides (congestion pricing and dynamic parking prices, for starters, Kahn suggests). And the real question for any community is whether those costs have grown so steep that they overwhelm all the benefits.

"For the vast majority of U.S. cities," says Berkeley economist Enrico Moretti, "we’re far from the point."

That's true of the Bay Area, he says, where a housing crisis has grown out of the fear that the region can't bear all of the people who want to live there.

"It’s obvious that we could have millions and millions of people in San Francisco if we built skyscrapers on every plot of land," Moretti says. "That’s not really interesting to talk about, because nobody wants that. I don’t want that. I don't think that's what's at stake."

What is at stake, he says, is all the housing the Bay Area could add without substantially altering its character, without turning it into Hong Kong. Just replace the surface parking lots, and the under-developed land near train stops, add a story or two here and there without paving over the scenic hills, and Moretti estimates the Bay Area has space for 30-40 percent more people.

In the city of Los Angeles, replacing single-family homes with modest four-unit buildings would make room for nearly 2 million new housing units, says Devin Bunten, a Ph.D. student working with Kahn. "It’s hard to think that we’d have a rent crisis facing our city," Bunten says, "if we allowed that kind of thing to happen."

The problem, Bunten argues, is that the costs of adding even some of those people (like less parking) are often visible right outside our doors, while the benefits (a more productive regional economy) tend to kick in at a larger geography. And, crucially, we make decisions about how to use land at this first scale, not the second one.

New housing permits are approved — and the Not-In-My-Back-Yard (NIMBY) types step to the microphone — in local planning meetings. And so, apparently, we're full. We can't spare a square. The neighborhood belongs to the people who are already here, and anyone who wants in will only threaten that.

"I find it deeply troubling on economic grounds," Moretti says, "because limiting housing supply is actually going to exacerbate the problem, not make it better. But also, it’s really a deep misunderstanding of the nature of cities. Cities always have been changing, throughout centuries. American cities have changed enormously in different and unexpected ways, and trying to build barriers around them is anti-urban in a deep sense."

This doesn't mean that no one has a right to live in a single-family home if they crave a yard and some privacy. It means that the person who lives in that home doesn't have a right to hold the world constant around it to preserve a parking spot. "To an economist, we’d ask 'did she buy it?'" Kahn says. "Why does that person have a right to easily finding a spot on her block?"

NIMBYs and inequality

For much of the 20th century in America, the ability to move from one place to another — from farming towns to manufacturing cities, from southern states to northern ones — was a crucial route the poor used to get their hands on opportunity. This happened, for example, during the Great Migration, and from the Dust Bowl.

Remarkably, researchers Peter Ganong and Daniel Shoag suggest that nearly a third of the decrease in economic inequality in the U.S. last century up to 1970 can be explained by workers moving from poor states to rich ones.

But since then something has changed. Home prices in California used to be about the same as home prices in Michigan or Connecticut. Today, there is a vast difference in housing costs across the country, and those differences, economists point out, have expanded in tandem with opposition to growth — and laws put in place to block it — in states like California.

It's now very expensive to live in those communities that have effectively declared themselves "full." And the larger implication is that this doesn't just force tech bros to rent thousand-dollar bunk beds in Silicon Valley; it also blocks one path to narrowing inequality for poor people who might move there, too.

William Fischel, who has long studied land use at Dartmouth, has some fascinating theories of what changed in the 1970s. For one, the national Environmental Protection Act empowered local residents worried about the environmental cost of new development. But the 1970s were also a time when we began to think of our homes not just as places to live, but as financial investments for the future.

And people who believe their homes will put their children through college, or fund their retirement, are bound to be more protective of them.

"That’s when people began to look around and say 'well what might threaten the value of this home?'" Fischel says of the 1970s. Residents were no longer worried about the old-school zoning concerns of keeping factories and housing apart. "They worried about 'well, if there’s another housing development down there, it’s going to crowd the schools, and create more traffic.' They became hyper-sensitive to community effects. And they started showing up in zoning hearings."

It's gauche to talk about property values at a public meeting, Fischel says, but the concerns people voice instead are clearly connected.

In his new book, "Zoning Rules!" Fischel cleverly uses a Google Ngram, which tracks the frequency of words in digitized books, to follow the rise of our obsession with housing values against the surge in interest in limiting growth. We've reproduced his experiment here:

Google Ngram Viewer

Apparently, it took us a little while to figure out what to call the people behind this trend:

Google Ngram viewer

This new attitude over the last 30 years has layered atop an American instinct that has always been here.

"It was part of the original promise that if you come from Europe to the U.S., you get your own space, with your own home, and it’s a private home, and it’s got elbow room from your neighbors," says Hirt, the Virginia Tech urban planning expert who has written a new book on the history of American zoning.

There's a deep-seated cultural perception of space in America, that we should all have a lot of it, that a town with a family per acre can be full. And it's not just because we have a big country; they have a lot of land in Russia, too, Hirt points out. Their cities are still much denser.

Of course, what's based on culture and politics – not physics — can change. So maybe we can learn to live differently.

“Everybody that lives in San Francisco thinks that San Francisco is the once-and-always great place," says McCarthy, head of the Lincoln Institute. "I was in San Francisco in 1971 and it wasn’t that great. It was actually in very bad shape."

The city was losing population then, and it took many years to recover. Now we think million-dollar micro-apartments are the norm.

"But 40 years from now, San Francisco might look like Detroit. And it might look like Detroit because people have decided to stop evolving and adapting," McCarthy says. "And instead we move around on the planet instead of making the places that we care about work.”