Not all first impressions have equal longevity. Which kinds of impression have the greatest mobility--downward and upward--over the course of acquaintanceships? Previous research has indicated that first impressions of extraversion (E) have greater longitudinal stability than first impressions of other Big Five traits: agreeableness (A), conscientiousness (C), emotional stability (ES), and openness (O). In this article, we propose an inferential account of E impression stability. With data from field and laboratory studies, we provide evidence that positive impressions of A, C, and ES are especially vulnerable to small amounts of contrary evidence, whereas positive first impressions of E are more resistant to contrary information. Impressions of E demonstrated minimal susceptibility to negativity effects in a longitudinal study of college roommate impressions (Study 1), in a study of perceivers' implicit theories about different trait domains (Study 2), and in an experimental study of manipulated impression change (Study 3).