1. A system comprising: a data access module configured to access search results data that is indicative of a plurality of items and that is indicative of ranking values of the plurality of items; a controller module configured to generate, for display on a client device, page data based on the search results data, the controller module being configured to select a first item from the plurality of items; and a diversification module, including one or more processors, configured to: access a first rule that is indicative of a first diversity characteristic of the page data; based at least on the first rule, determine a deviance value of a second item from the plurality of items; and based at least on the ranking values of the first and second items, determine a reordering-cost value of the second item, the controller module being configured to select, based at least on a first combination of the deviance and the reordering-cost values of the second item, the second item for placement ahead of the first item on the page data.