"But, of course, that's the issue," Lenard said. "It is highly likely the DNT mechanism would interfere with those benefits. Furthermore, the DNT mechanism cannot be compared to the popular do-not-call list, which reduces unwanted marketing messages. A DNT mechanism wouldn't necessarily reduce advertising messages, it just would likely make them less useful."

The FTC should compare the benefits and costs of do-not-track before making such a "major proposal," Lenard added.

The FTC is not yet calling for do-not-track legislation in Congress, but Web browser makers and other Internet companies should act quickly to implement a universal do-not-track list, Leibowitz said.

New privacy protections, either from industry or government, are needed, because Internet users are often confused about how companies are collecting and using their personal data, Leibowitz said. "Many companies are not disclosing their practices," he said. "Even among the companies that do disclose them, those disclosures are often done in long, incomprehensible privacy policies and user agreements that consumers don't read, let alone understand."