Turning off third-party tracking cookies costs websites close to a third of their advertising revenue, and the privacy-friendly technology Google built to replace them recovers only a sliver of that loss. That is the result of a large randomised field experiment on Chrome users.
The study, by Zhengrong Gu, Garrett A. Johnson and Shunto J. Kobayashi of Boston University’s Questrom School of Business, was published in the Proceedings of the National Academy of Sciences on May 5, 2026.
An open experiment overseen by a regulator
The researchers used an industry-wide test jointly overseen by Google and the UK Competition and Markets Authority. Chrome users were randomly assigned to browse with third-party cookies on, with them off, or with Google’s Privacy Sandbox replacing them.
They combined that random assignment with proprietary data from the ad management firm Raptive, covering more than 200 million ad impressions across over 5,000 publishers worldwide.
A 29% hit with only 4% recovered
Removing third-party cookies reduced publisher advertising revenue by 29.1%. Privacy Sandbox recovered just 4.2% of that lost revenue.
The authors note that this figure reflects how widely the industry had adopted the new tools during the study and how well they performed at the time, and that adoption was modest.
Privacy-preserving auctions also slowed ads down. The gap in median latency between the Sandbox and normal groups was 0.41 seconds on desktop, 1.41 seconds on smartphones and 1.82 seconds on tablets, which meant fewer ads were delivered and revenue fell further.
Johnson served as an outside expert for the Competition and Markets Authority in its Privacy Sandbox case, in his personal capacity. Beyond the cookie question, the authors argue that open, market-wide experiments like this one give regulators a way to measure the effects of platform changes without relying on the platform’s own data.
Study Details:
- Title: Can privacy technologies replace cookies? Ad revenue in a field experiment
- Authors: Zhengrong Gu, Garrett A. Johnson, Shunto J. Kobayashi
- Journal: Proceedings of the National Academy of Sciences
- Publication Date: May 5, 2026
- DOI: 10.1073/pnas.2603752123