The finding is highlighted in a new research paper by University of Illinois law professor Paul J. Heald titled “How Copyright Makes Books and Music Disappear (and How Secondary Liability Rules Help Resurrect Old Songs).”

The reasoning is fairly simple, despite strained efforts by the copyright industries to make it seem more complicated: After the passage of some time, works that are in the public domain, and therefore available to be published and marketed by anyone, are more likely to stay in the marketplace than are works that are owned, perhaps by someone who can’t even be found (see: orphan works). A particular copyright owner (such as a big publishing house with stockholders breathing down executive necks) might not see sufficient value in a given copyright at a given moment. But if a work is available to all, it’s far more likely that someone, or maybe lots of someones, will find it worthwhile, and potentially profitable, to publish it.

As Heald points out, copyright owners spend a lot of time and money trying to convince everyone policymakers that longer copyrights tend to provide the “incentive to create” that is central to copyright laws (contrary to widespread belief, copyright doesn’t exist to benefit businesses — it exists to benefit the public.) But Heald’s study shows that the incentive to create requires a relatively short copyright life. Once the big money has been made, copyright ownership is often of only marginal benefit to the owner. If the margin is deemed too small to invest in distribution of a work, the public is deprived of that work until the copyright runs out.

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And that is a really long time. When Congress extended the life of copyrights in 1998 (from the life of the author plus 50 years to the life of the author plus 70 years), it relied heavily on the theory, pushed by media companies, that the longer rights would somehow strengthen the incentive to create. Heald’s study shows this to be piffle. Meanwhile, the longer copyrights have sabotaged the core reason copyrights exist. “Copyright correlates significantly with the disappearance of works rather than with their availability,” Heald writes. “Shortly after works are created and proprietized, they tend to disappear from public view only to reappear in significantly increased numbers when they fall into the public domain and lose their owners.” Of course, proponents of longer copyrights are simply trying to protect their control over those few works that they can still wring money from.

Heald’s findings, based on a systematic study of availability of new (as opposed to used) books on Amazon, revealed the immense difference between books published in the 1850s and those from the 1950s despite the fact that many more books were published in the latter decade. And, as a chart included with his study starkly reveals, there is an “eye-poppingly disproportionate number of new Amazon books initially published before the public domain cut-off date of 1923 and new Amazon books initially published after 1923.”

Meanwhile, copyright lobbyists “argue — without empirical support — that bad things happen to the work when it falls into the public domain. The public interest, so the story goes, requires term extension to prevent a public domain calamity.”

Those lobbyists pin their arguments on theories that make no economic sense: essentially, that the market for media products like books, films, and music needs to be a monopoly in order to function. If anybody can market a given novel or film, they argue, then too many people will produce copies, driving down the price. But as Heald notes, despite “potential competition, exploitation will occur, just as it does in other markets where no one has a monopoly over the object of exploitation, e.g. the markets for string, milk, and pencils.” And none of those things, it should be noted, are protected with exclusive sale rights for any amount of time, much less for decades-long stretches.

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Copyright of course is needed so that originators can earn back their costs and make a decent profit — that’s what provides the incentive to create. But that doesn’t mean rights-holders should be able to hold their rights to the point where works are actually removed from the marketplace.

Heald’s paper also looks at how the “safe harbor” provisions of the Digital Millennium Copyright Act help keep older songs in circulation on YouTube (GOOG). YouTube offers an easy way for copyright holders to make money from the songs they own, even when they’ve been uploaded by infringers, and thus “satisfy the market of potential listeners” even when the songs aren’t otherwise available.