Dunning, D., Meyerowitz, J. A., & Holzberg, A. D. (1989). Ambiguity and self-evaluation: The role of idiosyncratic trait definitions in self-serving assessments of ability.
Proposed that such assessments occur because the meaning of most characteristics is ambiguous, which allows people to use self-serving trait definitions when providing self-evaluations. Studies 1 and 2 revealed that people provide self-serving assessments to the extent that the trait is ambiguous, that is, to the extent that it can describe a wide variety of behaviors. Study 3 more directly implicated ambiguity in these appraisals. ...