Internet filtering around the world has grown in scale, scope, and sophistication in recent years. These maps (see multimedia box at left), based on a study by an academic consortium, describe the extent to which nations block or restrict online content ranging from political dissent to porn. “Over the course of five years, we’ve gone from just a few places doing state-based technical filtering … to more than two dozen,” says John Palfrey, executive director of the Berkman Center for Internet and Society at Harvard Law School.

The OpenNet Initiative–a collabo­ration among researchers at Cambridge, Oxford, Harvard, and the University of Toronto–carried out its study in 2006 and early 2007 using technical tools that test filtering. The group also used reports from local researchers in some countries. Of 41 nations tested, 25 were found to block or filter content to various extents.

China, Iran, and Saudi Arabia remain top blockers, stamping out porn, political, human-rights, and religious sites. Other countries target specific categories: for example, Libya filters political content. In western nations, the story is more nuanced: U.S. libraries block some sites, and private parties remove copyrighted material to avoid lawsuits; in Germany, Nazi sites are banned. See opennet.net for details.