SABMiller and Anheuser-Busch InBev, the world's largest brewers, announced plans to merge on Wednesday. (Photo illustration by Scott Olson/Getty Images)

If any brand can claim to be a true American icon, it's surely Bud Light. A can of Bud, in fact, is almost as American as a free-and-open market in which firms compete to offer high-quality products at low prices.

But today, control of the market for beer in the United States is concentrated in just a few big brewers, and the industry's critics worry they have enough power to shut out the competition.

The beer industry underwent substantial consolidation in 2008, when Anheuser-Busch, the maker of Bud Light, was bought by InBev, a Belgian company. Around the same time, SABMiller of London and MolsonCoors of Denver merged their U.S. operations. Those two companies brewed other popular American beers, Miller and Coors.

Now, a new economics paper suggests all that deal-making may have increased the price of those companies' most popular brands.

In late 2008, the price of those beers increased sharply, from below $10 to nearly $10.40 for a 12-pack. There is no definitive explanation for the change, but one theory is that the newly combined MillerCoors and its freshly conglomerated Belgian rival, called Anheuser-Busch InBev or ABI, might have coordinated their prices to charge consumers more.

"The paper supports the increasing concern that highly concentrated industries result in higher prices from anti-competitive coordination," said Diana Moss, an economist who advocates for more competition as the president of the American Antitrust Institute.

The data on beer "should not be a surprise," she said. "There's no one else to inject any competition into the market."

ABI is the world's largest brewer, and it completed an agreement Wednesday to buy the world's second largest, SABMiller. The price tag is $108 billion. Together, the two brewers currently control 31 percent of the global market, according to the research firm Euromonitor. SABMiller owns a stake in MillerCoors, which brews Miller beers in the U.S..

MillerCoors was established on June 30, 2008. Prices for the most popular beers had been declining gradually up until that summer, according to two economists from Georgetown and Drexler universities, who analyzed a proprietary database of tens of thousands of transactions at cash registers in groceries and pharmacies.

That fall, prices for Miller Lite and Coors Light (both brewed by the newly merged company), as well as Bud Light (now brewed by its new owner, ABI), increased 6 percent simultaneously and remained at that level. Meanwhile, competing brands -- Heineken and Corona -- continued the decline in price. You can see that in these charts: On the left, Miller, Coors and Bud all surge in price, while on the right, Corona and Heineken decline.

The chart on the left shows the price in dollars, adjusted for inflation, of Miller Lite, Coors Light and Bud Light, which jumped by 6 percent in 2008. (Courtesy of Nathan Miller and Matthew Weinberg, who analyzed data from the IRI Academic Database.)

This change in price presented the economists, Georgetown's Nathan Miller and Drexel's Matthew Weinberg, with a puzzle.

Before Molson Coors and SABMiller joined forces to form MillerCoors  in 2008, Molson Coors had only two U.S. plants. One was in Golden, Colo., and another was in Elkton, Va. After the merger, the combined company could fill Coors cans at any of SABMiller's half-dozen breweries around the country. The merger should have reduced the costs of shipping and keeping beer cold, and those reduced costs theoretically should have been passed on to consumers. They weren't.

It is a puzzle without an obvious solution. Maybe the price of diesel increased that year, making shipping more costly and forcing the companies to raise prices. Yet expensive fuel would have also forced Corona and Heineken to increase their prices, and those brands became steadily cheaper.

Suppose MillerCoors, immediately after the merger, just happened to get unlucky on a contract for hops harvested in the 2008 season, forcing the firm to raise prices. That wouldn't explain why prices also increased for Bud Light at the exact same time and by the exact same amount.

And if brewing and shipping beer did become more expensive in 2008 for some reason, that doesn't explain why prices for Bud, Miller and Coors did not continue to decline in subsequent years, as they had before.

"The price increases that we estimate exceed the predictions of a standard economic model, accounting for changes in demand and cost conditions. This is consistent with tacit collusion between ABI and MillerCoors that emerges after the consummation of the MillerCoors joint venture," Miller and Weinberg wrote in response to questions.

"As is often the case with economic analyses, some alternative explanations cannot be ruled out solely on the basis of the data," they added.

ABI and SABMiller have argued that their pending merger will not affect the U.S. market at all, since SABMiller will sell off its U.S. business before ABI purchases it.

SABMiller's stake in MillerCoors will go to Molson Coors, which will then control MillerCoors entirely. After the sale, ABI will be able to acquire SABMiller without making the U.S. brewing industry less competitive, the companies argue.

"The draft academic paper released earlier this year focuses on a theoretical economic analysis that is unrelated to AB InBev’s combination with SABMiller," a spokesman for ABI wrote in a statement to Wonkblog. "Consumers have more choices today than ever before and nothing in the agreements we announced yesterday will change that."

This arrangement suggests that ABI may have learned from its last merger, when it acquired Grupo Modelo. Before ABI bought that company, federal regulators demanded that Grupo Modelo sell off its U.S. business.

To force Grupo Modelo to comply, the Department of Justice filed a lawsuit against ABI. The suit alleged that the proposed deal would result in illegal concentration, and that the major firms were already coordinating their prices. According to the complaint, ABI would decide on regional prices for the upcoming year every summer. MillerCoors would follow suit, instead of competing with its rival by reducing its prices -- although there was not evidence of an explicit agreement to fix the price.

That suit was settled when Grupo Modelo divested its U.S. operations, and the acquisition proceeded.

Defending their latest proposed merger, ABI representatives also argue that beer is a competitive market.

"Craft beers are growing rapidly, and the beer industry faces ever-increasing competition," João Castro Neves, ABI's president for North America, wrote in a letter to members of Congress Wednesday.

Yet Miller and Weinberg's findings do not provide evidence that consumers paid less due to competition from other breweries, wineries or distilleries. SABMiller and ABI were able to maintain their increased prices following the creation of MillerCoors.