Did Apple's App Tracking Transparency framework harm the app ecosystem?

We study the impact of Apple's App Tracking Transparency (ATT) framework on the Apple App Store ecosystem. ATT restricted developers' access to personal identifiers used for targeted advertising. Promoted as a privacy-enhancing initiative, the change drew sharp criticism from advertising technology firms, who predicted that it would substantially harm the app ecosystem. We collect data on every app available on both the Apple App Store and the Google Play Store in the 18 months around the implementation of ATT. We use a difference-in-differences analysis to comprehensively investigate whether the introduction of ATT negatively affected the app ecosystem. We analyze multiple possible downstream effects including developers' engagement with the ecosystem-measured through entry, exit, app updates, and adoption of Software Development Kits (SDKs)-and on consumers' valuation of apps, proxied by the volume and average rating of reviews. We find that the number of available apps on the Apple App Store quickly rebounded after an initial drop, driven by a temporary decline in entry rather than an increase in exit. In terms of use of Software Development Kits (SDKs), developers slightly reduced their reliance on Monetization and Ad Mediation SDKs and modestly increased their use of Authentication and Payment SDKs. These adjustments did not meaningfully affect consumer valuations of apps. Existing apps experienced a small decline in ratings received, while new entrants received slightly more ratings in their first month. Overall, our results suggest that, contrary to pessimistic predictions, ATT did not trigger developer withdrawal but instead prompted adaptation to a more privacy-protective regime.