Methods, systems, and apparatus, including computer programs encoded on a computer-readable storage medium, for serving content to a user. A method includes: receiving a request for content from a user, the content to be displayed in a slot associated with an online resource; determining an expected activity score for the user based at least in part on one or more criteria associated with a context of presenting the online resource to the user; comparing the expected activity score for the user to a threshold; selecting one or more content items to serve to the user based at least in part on the comparison; and serving the selected content items to the user responsive to the request.