Systems, methods performed by data processing apparatus and computer storage media encoded with computer programs for receiving feedback from a client device at which a presentation page of content items is displayed, the received feedback relating to an affirmative act performed by the user, determining, based at least in part on the received feedback, whether the user is likely to perform a predetermined action relative to one or more of the content items, based on a result of the determining, selectively identifying one or more replacement content items and transmitting the one or more identified replacement content items to the client device for display in the presentation page in place of one or more of the plurality of content items.