How to Train Your YouTube Recommender

YouTube provides features for users to indicate disinterest when presented with unwanted recommendations, such as the ``Not interested'' and ``Don\'t recommend channel'' buttons. These buttons are purported to allow the user to correct ``mistakes'' made by the recommendation system. Yet, relatively little is known about the empirical efficacy of these buttons. Neither is much known about users' awareness of and confidence in them. To address these gaps, we simulated YouTube users with sock puppet agents. Each agent first executed a ``stain phase'', where it watched many videos of one assigned topic; then it executed a ``scrub phase'', where it tried to remove recommendations of the assigned topic. Each agent repeatedly applied a single scrubbing strategy, which included disliking previously-watched videos or deleting them from watch history, as well as clicking the ``not interested'' or ``don\'t recommend channel'' button on newly-recommended videos. Overall, we found that the stain phase significantly increased the fraction of the recommended videos on the user\'s homepage dedicated to the assigned topic. For the scrub phase, using the ``Not interested'' button worked best, significantly reducing such recommendations in all topics tested, on average removing 88\% of them. Neither the stain phase nor the scrub phase, however, had much effect on videopage recommendations (those given to users while they watch a video). We also ran a survey ($N$ =300) asking adult YouTube users in the US whether they were aware of and used these buttons before, as well as how effective they found these buttons to be. We found that 44\% of participants were not aware that the ``Not interested'' button existed. However, those who were aware of this button often used it to remove unwanted recommendations (82.8\%) and found it to be modestly effective (3.42 out of 5).