1. A computer-implemented method comprising: 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 the proportion is greater than a specified threshold; in response to determining that the proportion is greater than the specified threshold, storing data that indicates an affinity between the first characteristic and the second characteristic; detecting a particular user's selection, from a list of items, of a particular item that possesses the first characteristic; determining, based at least on the data that indicates the affinity between the first characteristic and the second characteristic, a set of other items that possess the 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.