1. A computer-implemented method comprising steps of: determining a proportion of users who select both (a) items that have a first characteristic and (b) items that have a second characteristic; determining whether said proportion is greater than a specified threshold; in response to determining that said proportion is greater than said specified threshold, storing data that indicates an affinity between said first characteristic and said second characteristic; detecting a particular user's selection, from a list of items, of a particular item that possesses said first characteristic; determining, based at least on said data, a set of other items that possess said second characteristic; and presenting one or more items from the set of other items to the user as recommended items; wherein the steps are performed by one or more computing devices.