1. A computer-implemented method for predicting an opinion, the computer-implemented method comprising: building a corpus which includes a first collection of documents attributable to a first person and a second collection of documents not attributable to the first person, wherein the second collection of documents is identified from content in the first collection of documents, wherein each of the second collection of documents is available from a respective secondary source; evaluating the corpus to build a model representing opinions of the first person relative to topics, concepts, or subjects discussed in the first and second collections of documents, including determining, for each secondary source, a source weight factor characterizing a presumed opinion of the secondary source held by the first person; receiving a request to predict an opinion of the first person regarding a topic specified in the request; generating a predicted opinion of the first person regarding the topic from the model, by operation of one or more computer processors and based on: (i) the opinions of the first person relative to topics, concepts, or subjects discussed in the first and second collections of documents and (ii) each source weight factor characterizing the presumed opinion of the respective secondary source held by the first person; and returning the predicted opinion responsive to the request.