Children with autism achieve mirror self-recognition appropriate to developmental age, but are nonetheless reported to have problems in other aspects of a sense of self. We observed behaviour in the mirror in 12 pre-school children with autism, 13 pre-school children with Down syndrome (DS) and 13 typically developing (TD) toddlers. Reliable differences in reflecting actions, social relatedness and positive affect towards themselves, and an absence of coy smiles differentiated the children with autism from the others. The children with DS showed the highest interest in their own faces. These differences were largely independent of mirror self-recognition (MSR), broadly supporting arguments for dissociation between interpersonal and conceptual aspects of self. Mirror behaviour may be a subtle but easily elicited measure of the social quality of a sense of self.