1. A method for inducing a top-down hierarchical categorizer, comprising: providing a set of labeled training items, each labeled training item including an associated label representing a single category assignment for the training item; providing a set of unlabeled training items; associating a prior with the set of unlabeled training items that is independent of any particular feature contained in the unlabeled training items, the prior representing a plurality of possible category assignments for the set of unlabeled training items; and inducing a top-down hierarchical categorizer with a machine learning algorithm based on the set of labeled training items, the set of unlabeled training items, and the prior.