Previous research has suggested that children of 5/6 years fail to understand that they are the authority on their own self-knowledge. That is, when asked questions like, 'Who knows best when you are feeling tired?', they tend to cite their mother rather than themselves. Here we report a study that, rather than asking about generalities (Who knows best what you are thinking?'), presented 5-, 7- 9- and 11-year-children with hypothetical vignettes about specific circumstances in which they were described as either disclosing or not disclosing a specified state to their mother. Children were subsequently asked to judge who would best know the state. Over all age groups children were significantly more likely to identify themselves as authorities on their own self-knowledge when states had not been disclosed to mother than when they had. However, in the case of disclosed states, young children (though not older ones) asserted that, 'mum knows best'. These findings are interpreted as suggesting not that young children entirely fail to understand first person authority, but instead that they make the relatively sophisticated assumption that mothers' interpretive competence is greater than their own.