An analysis by Brookings Institution scholars has found 606 incidents of governments interfering with some portion of their citizens' web content since 1995.
Overall, 45 percent of the incidents occurred in democracies or emerging democracies while 52 percent occurred in authoritarian regimes, according to the study titled The Dictators' Digital Dilemma: When Do States Disconnect Their Digital Networks?.
A significantly higher proportion of Web-interference incidents come from authoritarian and failing states after around 2000, likely a result of the earlier adoption of Internet technology in many democracies.
"We find that overall more democracies participate in network interventions than authoritarian regimes," the study's authors write. "However, authoritarian regimes conduct shutdowns with greater frequency."
Incidents cataloged by the scholars range from Egyptian President Hosni Mubarak's last ditch attempt in January to undercut street protesters calling for his ouster by shutting down the Internet, which they were using as an organizing tool, to a 1997 case in which a German court fined a CompuServe official for allowing sexual material on sites his company hosted.
National security was the most commonly cited reason for disabling parts of the Internet. Other reasons included preventing state secrets from being released and upholding public morals.
The only U.S. incident cited in the report is the 2009 arrest of activists at the Group of 20 meeting in Pittsburgh who were Tweeting about police locations.
The three countries with the highest number of incidents are China, Tunisia, and Turkey. China's "great firewall" is widely considered the most sophisticated Web censorship tool in the world. Turkey's Internet censorship is largely aimed at removing obscenities.
Old-fashioned web censorship has been surpassed in many oppressive regimes by what scholars call censorship 2.0 practices, such as distributed denial-of-service attacks that shut down websites by flooding them with more operations than they can handle and malicious patches that change a Web page's content.