What gives? Do sports teams already have all the space they need? Have taxpayers grown reluctant to finance these monuments to the vanity of billionaire owners?

We called economist J.C. Bradbury, who helped build a database of all 220 major sports facilities constructed in North America since 1909, updating the data that Judith Grant Long gathered for her 2014 book. Billionaire owners aren’t always forthcoming, so they often base their work on “ballpark” estimates from press accounts and other public sources.

“It’s purposefully, in my opinion, obfuscated from taxpayers,” especially in more controversial cases, said Long, a professor of sports management and urban planning at the University of Michigan who first assembled the data for her PhD dissertation in the early 2000s.

Bradbury, who updated Long’s data from his perch at Kennesaw State University, outlined two great waves of sports construction. The first hit in the 1960s as television brought sports to the masses, revenue rose and newly expansionist leagues sprawled across the country.

Those first “super stadiums” were cavernous concrete buckets meant be filled with multiple sports and events — think Houston’s Astrodome or RFK Stadium in the District. Many were built with public funds and envisioned as public resources.

The second wave hit in the late 1990s: An incredible 56 facilities rose from 1995 to 2004 as owners realized they could tap into fresh fire hydrants of money by swapping their generic sports buckets — most still perfectly functional — for venues tailored to specific sports and larded with restaurants, clubs and luxury suites.

The cost to build those sports spaces more than doubled during that second surge of construction even after adjusting for inflation, from a median of $190 million in the 1980s to around $480 million in the 2000s.

Costs have tripled since the 2010s as facilities become more opulent. Much of that increase has fallen on team owners. But the median public subsidy for an arena or stadium has also grown steadily, from $122 million in the 1980s to $500 million since 2020.

What is the public actually paying for? For the answer, we turned to Geoffrey Propheter, a University of Colorado Denver economist who dredged up more than 100 lease agreements for his book, “Major League Sports and the Property Tax.” Propheter said today’s sports team leases are “complex legal artifacts” with hundreds of pages detailing byzantine financial arrangements that somehow always manage to lower owners’ operating costs and/or their tax burdens.

If you were working on one of these deals, your first move might be to take a chunk out of your property tax bill by giving the dirt under the stadium to the local government, making it — voilà! — untaxed public land. In some places, you would still owe property taxes on the building above the land and on the value of your temporary possession of the land over the term of your lease. But maybe not! Lawmakers might exempt you entirely or count your property tax payments as credit toward rent.

You might even give the building to the local government as soon as the lease is up, when its most profitable days are behind it, leaving taxpayers with “a giant paperweight,” Propheter told us. “Now they’ve got to do something with this pile of concrete and steel,” especially if the lease includes a noncompete clause with a new arena or stadium — and that something might be demolition.

Propheter’s data shows sports team leases, like bell-bottom pants and confused cicadas, are on a roughly 30-year cycle with nearly three-quarters lasting between 25 and 40 years. Since the last sports building boom started around 1995, we could be staring down the barrel of another construction wave: The leases of about 44 teams across four different leagues will expire in the next decade.

If the majority of those team owners get new facilities, it could produce one of the greatest stadium-construction frenzies in modern history, easily surpassing the Y2K era in sheer dollar terms. Even renovations can have a stunning price tag: The overhaul of Capital One Arena — built for $200 million in 1997 (about $385 million in today’s dollars) — is set to receive a $515 million infusion from D.C. on top of the more than $200 million Leonsis has paid to upgrade the arena since 2014.

You might wonder: Do we need new stadiums? Is something wrong with today’s ballparks?

Not really, unless you consider not raking in as much money as humanly possible to be a defect.

A new stadium ignites what economists call the novelty effect, as interest in the new digs enables owners to crank up ticket prices. Revenue soars in the first few years and remains higher than normal for a decade. A new stadium also lets you copy all the profit-making mechanisms your competitors invented in the decades since you last built a facility, such as spendy dining options and luxury suites with wall-consuming televisions.

The latest trend seems to be sprawling mixed-use developments that promise to create urban entertainment hubs, such as the Battery Atlanta around Georgia’s Truist Park. According to Long, owners are using venue construction “as a Trojan horse … to control larger swaths of land.” By unlocking powerful real estate development tools, a new stadium allows a team owner to create a broader development that captures even more revenue — which, in this case, once went to ordinary barkeeps and restaurant owners hoping to serve the game-day crowds.

“This is often pitched as additional economic development impact,” said Nathan Jensen, a University of Texas at Austin subsidy expert and technically an NFL owner: He grew up in Wisconsin and owns a single share of the Green Bay Packers. But as a result, “people going out for a beer before a game are captured by the developer and are subsidized.”

We may be seeing basic economics at work. New stadiums typically enjoy hundreds of millions of dollars in incentives from local governments. And when you subsidize something, you get more of it, whether you want it or not. Propheter has found that subsidized facilities also tend to be more opulent than their private peers.

Are those subsidies a wise economic investment? Reams of research show that new sports venues don’t generally create promised economic booms. A massive analysis of 42 years of professional sports teams and facilities found that the overall sports environment had an impact on wages — but, uh, not always a positive one. Data on employment and sales found similar results. For example, restaurants and bars near Chesapeake Energy Arena in Oklahoma City benefited from their new neighbor, but others — including nearby entertainment businesses — suffered.

The reality is that money spent on sports doesn’t come out of thin air. It is money that fans might have spent elsewhere. Arenas and stadiums can revitalize a neighborhood by pulling spending from other parts of town, but that’s different from creating new economic activity. While every ownership group argues that their new facility will rejuvenate half the city and make a profit for taxpayers, research shows that sports subsidies simply do not generate the kind of economic benefits they promise to the public.

According to Long, predictions about job creation and sales tax revenue tend to come from the same handful of consultants reusing the same methods that have been inaccurate in the past. On top of that, teams often lowball their estimates of construction costs by covering only part of the true public price tag, leaving out unsexy essentials like sanitation services or transportation infrastructure.

Operating expenses add another wrinkle. Consider Barclays Center in Brooklyn, whose financials our new hero Propheter went through with a fine-toothed comb. Its developer, Forest City Ratner, predicted the arena would make a profit of about $35 million annually. In its first three years, revenue actually beat expectations. But Forest City Ratner’s forecasts dramatically underestimated the arena’s operating and debt-servicing costs, which were about twice as high as expected, driving profits down from $35 million to a maximum of $6 million per year.

So why do local officials keep shoveling out money for new stadiums and arenas? It’s partly that sports owners threaten to leave, as Leonsis did late last year, but it’s not just that. Teams have been known to get new facilities without another suitor waiting in the wings.

Data can’t really help here, but according to Bradbury, powerful people may just like sports.

“Politicians love two things: jocks and movie stars,” he told us. And it’s bipartisan: “Democrat and Republican can both agree, ‘We’ve got to have a stadium.’”

Hello there, Data Hive! The Department of Data craves questions. What are you curious about: How have major cities skylines changed over the decades? What are Wall Street’s biggest investors? How did our spending change after the coronavirus pandemic? Just ask!

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