No one would confuse cyber-lawyer Lawrence Lessig with Homer Simpson -- symbol for all that is dumb about America. But Lessig, a professor of law at Stanford University in Palo Alto, CA, has spent a good deal of time lately trying to figure out how to resolve a “doh!” moment of his own: realizing that the various intellectual property licenses that he helped design over recent years to encourage the sharing of intellectual property -- a movement collectively known as "copyleft," in contrast to copyright -- may actually prevent it.

The problem is that the copyleft licenses, like the "free software" licenses from which they're drawn, require that derivative works be licensed under identical terms. And those terms differ from license to license. So the collections (or "mash-ups") of free text, audio, and video that Lessig and others have championed as the vanguard of a new "free culture" can't combine works created under different licenses -- even if all of the licenses are meant to encourage wide sharing. For example, text from Wikipedia, which uses a license called the Free Documentation License (FDL), could not be used as narration in a documentary film designed to be shared under a Creative Commons license.

In 2001, Lessig was one of the founders of Creative Commons, which was started to create flexible versions of copyright that would make it easier to share elements of creative works, such as text, video, and audio. The idea came out of the free software movement spearheaded by former MIT computer scientist Richard Stallman, who developed the free GNU operating system. Under Stallman's definition, software is free if it can be modified and distributed freely. The best-known example of free software is Linux, a version of GNU that includes a core or "kernel" written by Linus Torvalds and his followers.

People who want to share their creative works under more flexible terms than those allowed with traditional copyright can now choose from six main versions of the Creative Commons license, ranging from the permissive “attribution only” license, which allows anyone to use your creation for any purpose, as long as you receive credit for it, to the “attribution noncommercial no derivative” license, which allows reuse, but not modifications or commercial use. Creative Commons also has several niche licenses covering things like audio sampling.

Although Creative Commons does not track how many writers, artists, and others are using its licenses, it does count "linkbacks" to its site -- many of which come from the sites of writers, publishers, or artists using the group's licenses. There are now 45 million such linkbacks, up more than tenfold from a year ago.

But the Creative Commons licenses are not the only copyleft licenses in widespread use. The FDL, used by Wikipedia and written by Stallman's Free Software Foundation, differs from the Creative Commons not only in language but also in intent; it was designed primarily to protect manuals accompanying free software, and applies mainly to textual information.

Like the Creative Commons license, it requires that derivative works be distributed under the same terms -- which can be burdensome even if one isn't doing a mash-up. “If we had a photo on the site and you wanted to use it to make a postcard, you’d have to put the license on the back, and it runs eight pages,” says Jimmy Wales, co-founder of Wikipedia and chair of the board of Wikimedia in St. Petersburg, FL.