The Manhattan skyline, as seen from the top of the Rockefeller Center, courtesy of Flickr user Katie Haugland under a Creative Commons license.

Jason Barr is an economist who studies skyscrapers. Or, as he jokes, skynomics: how construction costs and condo prices shape the skyline, how building booms chase business cycles, why new record-breaking towers reach the heights they do.

"Fundamentally," he says, "the skyline is a rational result of underlying economic forces." Look at the city that way, and architecture reflects the booms and busts of the American economy over the last century. A timeline of skyscraper construction in New York City in particular mirrors the exuberance of the Roaring '20s, the spending standstill of World War II, the rowdy years on Wall Street and the hiccups of crises from oil spikes to the recession.

This graphic of New York skyscraper construction, from the Council on Tall Buildings and Urban Habitat and capturing some of Barr's research, reveals much of that history (click here for an interactive version):

That picture — which looks a bit like a skyline itself — shows both the number of skyscrapers (buildings more than 100 meters tall) completed in New York each year and the height of that year's tallest building. The timeline starts in the early 20th century, when the inventions of the steel frame and the elevator first made building truly tall possible. Since then, there have been clear cycles in skyscraper construction — a rise and fall in new skyline additions — typically about a quarter-century long.

"[Economic growth] is going up and down, but underneath that are these larger currents, whether it's technological changes, or the demand to be in New York City, or the demand for new modern office space," says Barr, an associate professor at Rutgers University-Newark, who has a book out this spring, "Building the Skyline: The birth and growth of Manhattan's skyscrapers."

In the timeline above, the 1920s are a time of massive new skyscraper construction in New York, culminating in 1931, the single most active year for skyscraper construction in city history (32 towers were completed that year). That building boom reflects in part what Barr calls the "office-ification" of the American economy, the beginning of the shift to a modern economy where more workers moved out of factories and into desks.

"New York City is the eye of the storm in that sense — it’s America’s urban center," Barr says. "There’s just this tremendous need for office space to house the growing modern economy. Any company that had aspirations to be a national, international company, they had to have some office presence in New York."

Skyscraper completions plummeted after 1931, with the onset of the Great Depression. And then not a single new skyscraper was built through the end of World War II, with construction materials scarce.

Another skyscraper cycle sets in after the war. The rise of multinational corporations pushes the demand for new and more modern office space into the mid-1970s. Another boom in the '80s coincides with the manic years on Wall Street. There's little new construction in the mid 1990s after all the overbuilding over the previous decade. Other construction slumps occur at predictable times: in the late 1970s, and again briefly after the Great Recession.

[In the shadows of booming cities, a tension between sunlight and prosperity]

The current boom now remaking New York's skyline has largely been driven by luxury condos, not new offices. And it reflects another pattern Barr has detected over the years: Periods of extreme income inequality in the U.S. have occurred alongside the growth of New York's skyline. The super-tall towers rising today, with their multi-million-dollar living-room views of Central Park, mark a clear convergence of architecture and inequality, the very embodiment of "underlying economic forces" seen in glass and concrete.

Past skyscraper cycles in New York's history would seem to suggest that we're approaching another peak — and decline. But, Barr cautions, he can't make predictions about the economy by looking at the spires that are rising today. The skyline is remarkable for what it tells us about economic history, not the economic future.

The completions above tend to lag behind events in the economy by about two years, given the time it takes to construct these buildings. That means indicators in the economy — unemployment, trading on the New York Stock Exchange — hint at what may be built in the coming years, not the other way around.

"When is this cycle going to end?" Barr says. "When there’s a huge downturn on Wall Street, when people stop trading, when money becomes expensive. When are all the forces going to turn against New York City to really put a damper on the market? We don’t know."