Mentalization theory concerns not only impaired mentalizing development but also normal development and potentially expertise. This study aimed to compare three groups contrasted on the basis of their hypothesised mentalizing capacities. Specifically the mentalizing of self and of others; hypothesised to vary somewhat independently, were measured separately using a newly developed self report measure; the Reflective Function Questionnaire. Additionally four measures of capacities allied to self and other mentalizing were administered. In a cross-sectional questionnaire-based design participants recruited were either individuals with borderline personality disorder (BPD) (‘poor’ mentalizers) in specialist clinical settings, non-clinical controls in cafes or Psychological Therapists (‘expert’ mentalizers) in training courses or professional forums. In self-mentalizing the BPD group scored lower than controls whilst controls scored lower than Psychological Therapists. In mentalizing others no significant differences were found between groups. Self-other profiles for the BPD group and Psychotherapists were explored with reference to the control group. The BPD group profile was characterised by differential impairment on self and other and impairment in self-mentalizing. However low cognitive empathy scores in comparison to the other groups suggested this required further investigation. The Psychological Therapist profile was characterised by enhanced mentalizing of self. Results are discussed in relation to the developmental theory of mentalizing, including the potential for developmental deficits to interact with more active defensive operations to impair mentalizing of self and others differentially. Psychological Therapists’ enhanced mentalizing capacities may include the ability to inhibit rather than enhance the impact of others’ emotions on the self.