Self-enhancement is linked to psychological gains (e.g., subjective well-being, persistence in adversity) but also to intrapersonal and interpersonal costs (e.g., excessive risk taking, antisocial behavior). Thus, constraints on self-enhancement may sometimes afford intrapersonal and interpersonal advantages. We tested whether explanatory introspection (i.e., generating reasons for why one might or might not possess personality traits) constitutes one such constraint. Experiment 1 demonstrated that explanatory introspection curtails self-enhancement. Experiment 2 clarified that the underlying mechanism must (a) involve explanatory questioning rather than descriptive imagining, (b) invoke the self rather than another person, and (c) feature written expression rather than unaided contemplation. Finally, Experiment 3 obtained evidence that an increase in uncertainty about oneself mediates the effect.