Harvard University professor Jonathan Zittrain supports some sort of global do-not-track system because "there are currently no functioning limits on what data gets collected and how it is used, and the rise of cookie consortia like Doubleclick means otherwise-unrelated Web sites can all quietly serve as collection points for data about us that gets fed to a central source. If kept for long periods of time and not distilled, that data can prove as revealing about us as, say, our search engine histories." But he worries that making opt-out too easy for people could "injure the model that's bringing them free content."