Patents are designed to help innovative people and companies protect their ideas. But a research paper from the Boston University School of Law argues that a growing wave of patent litigation is costing the economy dearly - as much as half a trillion dollars in lost wealth over the past two decades.

“It’s a tax on innovation,’’ said coauthor Michael Meurer, a BU Law professor.

The research examined lawsuits filed by “non-practicing entities,’’ or NPEs - companies that don’t produce products, deliver services, or invent things of their own. Instead, they buy patents from other companies or individuals and try to collect licensing fees, often through litigation, from companies they say are infringing on those patents.

Critics, including Meurer, call such firms “patent trolls,’’ likening them to creatures from Norse mythology who hid under bridges and collected steep tolls from travelers.

Patent lawsuits from such entities date to the 19th century. But back then, patents generally protected precisely defined inventions, such as the molecular composition of an industrial chemical. In recent years, the government has issued patents on innovations that are harder to pin down as original creations, like computer software or innovative business methods. In one famous case, the Internet retailer Amazon.com patented the concept of using a single mouse click to place an order.

The result, Meurer said, is a horde of vaguely defined patents that can be interpreted so broadly that many major companies could be sued for violations. Firms specializing in patent lawsuits scoop up many such patents, demand that companies pay to license them, then file lawsuits against those that don’t comply.

“Judges are often sensible,’’ Meurer said, “and will often read these apparently broad claims narrowly. But not always.’’

Patent lawsuits can be damaging regardless of who wins.

When publicly traded companies are hit with such actions, their stock values may decline. Meurer and his colleagues studied 1,630 such lawsuits over a 20-year period, most involving software patents. More than 4,100 companies were targeted by NPEs, which often sued multiple companies with a single complaint. Meurer found that the stock of affected companies lost a total of more than half a trillion dollars in value between 1990 and 2010. The losses in stock value amounted to an average of $83 billion a year in the last four years, or about one-fourth of US industrial research and development spending during the same period.

This would not be a problem if the money went to the NPEs that owned the patents; then there would be no net effect on the overall economy.

But Meurer found that less than 10 percent of the lost value goes to the companies that filed the lawsuits, or to the original inventors. Most of the wealth simply vanishes.

“A majority of the losses are ‘deadweight’ loss,’’ Meurer’s coauthor, James Bessen, wrote in an e-mail. “They don’t go to anybody. This includes sales that are delayed or canceled, investments not made, resources diverted to legal defense.’’

Historically, firms that held and defended patents were able to protect the financial interests of investors. Even now, an independent inventor who lacks the money for a patent suit against a giant company can sell the patent to an NPE for a profit, Meurer said. But with so much disappearing wealth, he added, the lawsuits cause a net loss to the economy.

The study did not sit well with Paul Ryan, chief executive of Acacia Research Corp., a major NPE in Newport Beach, Calif.

“I can’t believe this was done by professors,’’ he said. “This is like third grade. The day-to-day ups and downs of stocks have nothing to do with the social or private costs of the patent system in the United States.’’

Companies like his offer a vital service to independent inventors, Ryan said: “We represent America’s inventors, who have consistently been screwed by large companies for the past 50 years.’’

Ryan also said the study is biased, noting that it was partly funded by the Coalition for Patent Fairness, a lobbying group backed by Apple Inc., Google Inc., and other giant technology firms that want restrictions on patent suits.

Bessen said the coalition provided “at most a couple hundred dollars’’ to pay a research assistant. “It is pretty hard to see how that would bias our results,’’ he said.

Patent legislation recently signed into law by President Obama includes changes that could reduce the impact of NPE suits. Now, it is easier for a defendant to win by showing that his company was already using the technology in question before a patent was granted by government.

In addition, the law requires NPEs to file separate suits against each company that has allegedly violated a patent.

“That raises the cost to the trolls of enforcing their patents,’’ Meurer said.

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