Mark(ing) your own homework (where "your" is often substituted with "their", "one's", "its", "his" or "her") is a British expression used in political discourse, the study of organisational behaviour and in everyday life. The expression implies that whenever a person or group self-assesses and/or self regulates their own work they will usually treat it more favorably than if it were assessed by an independent person or group.
Examples
References
Marking your own homework Wikipedia(Text) CC BY-SA