1. A method implemented by one or more computing devices, the method comprising: assigning at least one category in a hierarchy of categories to each of at least two objects; and computing a similarity score for the at least two objects based on an effect that each category in the hierarchy has on other categories in the hierarchy, the similarity score computed based, at least in part, on: a distance factor that is a function of a distance in the hierarchy between the at least one assigned category of the at least two objects; and a siblings factor that is a function of a number of siblings in the hierarchy and measures how much the number of siblings of each of the categories affects influence of said category on the other categories in the hierarchy, the siblings factor increasing the influence of the at least one assigned category on ancestor categories when the at least one assigned category has a greater number of siblings.