1. A computer-implemented method performed by a data processing apparatus, the method comprising: receiving data reflecting characteristics for users in a set of users, the characteristics for at least some of the users including indications of endorsements of content items by the users; receiving one or more parameters for a first user filter; identifying a subset of the set of users satisfying the one or more parameters of the first user filter by comparing the parameters of the first user filter to characteristics of users in the set; determining, for each of a plurality of content items that are endorsed by one or more users in the subset, a difference between a first count of users in the subset that have endorsed the content item divided by a total number of users in the subset and a second count of users in the set that have endorsed the content item divided by a total number of users in the set; identifying several content items based on the differences; for each of the several content items, determining a score for the content item based on a first count of users in the subset that have endorsed the content item and based on a second count of users in the set that have endorsed the content item; selecting one or more content items based on the scores; and transmitting data specifying the selected one or more content items.