15. The system of claim 11 , wherein the acceptance history data for previous presentations of a candidate content item comprises: a content topic for the candidate content item; a document topic for a document with which the candidate content item has been presented; an indicator of a response to a presentation of the candidate content item; and at least one historic sentiment score.
15. The system of claim 11 , wherein the acceptance history data for previous presentations of a candidate content item comprises: a content topic for the candidate content item; a document topic for a document with which the candidate content item has been presented; an indicator of a response to a presentation of the candidate content item; and at least one historic sentiment score. 16. The system of claim 15 , wherein the at least one historic sentiment score is one of a score for a sentiment towards the content topic, a score for a sentiment towards the document topic, and a score derived from a score for a sentiment towards the content topic and a score for a sentiment towards the document topic.