This paper proposes that variance in interpreter performance is dependent on factors of both general cognitive ability and personality. Whilst there is no doubt of the interplay between individual personality traits and job performance across many occupations, the greatest interest lies in determining which traits play the most important role; and to what extent these variables impact on learning and achievement. The paper reports on a study of 110 accredited signed language interpreters in Australia. Psychological constructs of self-efficacy, goal orientation and negative affectivity were measured, as were interpreter ratings of self-perceived competence as practitioners. The most significant finding revealed the dimension of emotional stability (represented on the negative end of the continuum by traits of anxiety and neuroticism, and measured in this study by the negative affectivity scale) as a predictor of interpreter's self-perceived competence. Based on these findings, recommendations for admission testing and interpreter education curricula are discussed.