Interesting, if you combine the mentioned concepts of `the recommendation system seeing the user as the history of all the videos they've watched` and 'introducing diversity.' Is that why watch history only gets saved for X number of videos/months? The rationale being users change and so we want the recommendations to change with them. <br>Because I think technically saving an array video ids to window.localStorage or IndexedDB inside the user's browser wouldn't take up too much space. Or is there some other technical limitations I'm not aware of here?,(reply>), This would make sense, if it didn't just cycle through the same videos again and again(as seems to be the case). To me, this is the most annoying 'feature' of the algorithm- and just a small proof that the objective function is less focused on diversity or any of the user satisfaction metrics that we tend to use in our subjective evaluations. It wasn't clearly stated throughout the talk, but is the true objective anything other than maximizing some APM (adds per month) metric? Seems logical to assume that all  the goodies we experience are just the fortunate side effect, and the occasional repetitions- well that would just be the result of 'pruning' for that main objective..,(reply>), ​@Erdal Karaca True, but for the user there's no incentive to do that, and the impact of doing it (total # of browsers you have access to, ignoring the potential few click-farms) on average would be minimal. And finding a way to ensure it isn't modified wouldn't be hard either, via something like JWT and encrypting server-side.,(reply>), The browser's local storage areas could be manipulated, hence tricking the algorithm, couldn't it?