1. A computer implemented method for identifying spam documents in an information retrieval system, the method comprising: maintaining a list of phrases, each phrase associated with a list of related phrases; determining a number of related phrases expected to be present in a document for any phrase on the list of phrases; determining for a document, and for at least one phrase in the document, an actual number of related phrases present in the document; and identifying the document as a spam document by comparing the actual number of related phrases present in the document with the expected number of related phrases.