Commercial-Interest-Weighted User Profiles

Systems, methods performed by data processing apparatus and computer storage media encoded with computer programs for maintaining a user interest profile corresponding to a user and containing information describing visits to publisher sites (e.g., and/or individual pages within a site) over a predetermined period of time; analyzing each of the visited publisher sites in the particular user's interest profile to identify publisher sites that indicate a level of commercial interest; based on a result of the analyzing, assigning a commercial-interest weight value to each of the visited publisher sites such that publisher sites indicating higher levels of commercial interest receive higher commercial-interest weight values than publisher sites indicating lower levels of commercial interest; updating the particular user's interest profile based on the assigned commercial-interest weight values; and using the updated user interest profile to determine subsequent content items to be presented to the particular user when visiting publisher sites.