Disputes over patent policy often revolve around moral principles. Take software patents. Many programmers think software ideas should flow freely, untrammeled by the threat of lawsuits. Others, like Nathan Myhrvold, argue that infringement is just plain wrong.

To get a sense for the scale, Lee and Mulligan estimate that if every firm in America that uses software — from maintaining a Web site to using an Internet-based invoice system — wanted to check its code for infringement, it would take 2 million patent attorneys, working full time, to pore over the records. That would cost about $400 billion in lawyer’s fees. And, for reference, there are only around 40,000 software patent lawyers currently employed in America today. That’s why most developers don’t even bother checking for infringement. The costs are prohibitively high. Instead, they plow ahead with development and hope for the best.

This, of course, is excellent news for patent trolls. As a trio of Boston University researchers — James E. Bessen, Michael J. Meurer, and Jennifer Laurissa Ford — recently found, infringement lawsuits filed by companies that aren’t even using their patents (known as “non-practicing entities”) have skyrocketed since 1990, costing the economy $500 billion over that timeframe and depressing innovation.

Most of these suits involve vague business and software patents that aren’t always clear in scope or easy to understand. Many software patents, for instance, are filed for relatively common techniques that multiple programmers might reasonably think up. (To pick just one example out of a hat, IBM recently moved to patent the “well-known” concept of partitioning software to protect against malware.)

And what Lee and Mulligan try to make clear is that the very fuzziness of software patents often makes it practically impossible for a well-meaning developer to check to make sure he or she isn’t infringing in the first place. “It’s hard to predict which aspects of a software product someone might try to patent,” Lee explains at Ars Technica. “It’s even harder to predict which terms a patent lawyer might use to describe these concepts. So searching by keywords is likely to uncover only a fraction of relevant patents.”

The patent mess in America appears to be a downer for innovation. As an OECD report from last September concluded, “The quality of new patents around the world has fallen dramatically over the past two decades.” It’s doubtful that software patents are entirely responsible for this state of affairs. But our current legal regime doesn’t appear to be helping, either.