A method of inducing a top-down hierarchical categorizer includes providing a set of labeled training items. Each labeled training item includes an associated label representing a single category assignment for the training item. A set of unlabeled training items is provided. A prior is associated with the set of unlabeled training items that is independent of any particular feature contained in the unlabeled training items. The prior represents a plurality of possible category assignments for the set of unlabeled training items. A top-down hierarchical categorizer is induced with a machine learning algorithm based on the set of labeled training items, the set of unlabeled training items, and the prior.