1. A non-transitory computer readable storage medium having a computer program stored thereon, the program comprising instructions that, when executed by data processing apparatus, cause the data processing apparatus to perform operations comprising: analyzing a recent search activity period of a user to determine a short-term category of interest of the user, wherein the analyzing comprises comparing queries submitted by the user during the recent search activity period with categories of selected search results that were selected by the user during the recent search activity period; obtaining a plurality of search results responsive to a query, each of the plurality of search results having a respective score; for each particular search result of a first plurality of the search results: calculating a category selection value for the particular search result, wherein the category selection value for the particular search result is based on a measure of a count of selections of the particular search result as a portion of combined selection counts for search results responsive to the query for a plurality of users for the short-term category of interest; calculating a general selection value for the particular search result, wherein the general selection value for the particular search result is based on a measure of a count of selections of the particular search result as a portion of combined selection counts for search results responsive to the query for a plurality of users for any category of interest; calculating a category relevance for the particular search result, wherein the respective category relevance for the particular search result is based on a difference between the respective category selection value for the search result and the respective general selection value for the particular search result; selecting one or more search results of the plurality of search results, each selected search result being selected based on the selected search result having a category relevance that exceeds a threshold; adjusting the respective score for each of the selected search results based on, at least, the category selection value for each selected search result and the general selection value for each selected search result; and ranking the search results of the plurality of search results according to the respective adjusted scores for the selected search results and the respective scores for search results that were not selected.