This study proposed and tested a developmental model of impression formation based on observed behavior, prior expectancies, and additional incongruent information. Participants were 51 kindergartners, 53 second graders, and 104 college students who provided trait and liking judgments after watching a child actor engage in behaviors from three behavioral dimensions. Reaction times of judgments were also measured. Results showed that all age groups made dispositional attributions for all three presented traits. Nevertheless, young children had some difficulty conceiving of others as possessing both positive and negative attributes and provided less negative judgments. Prior expectancy strengthened judgments of the expected trait but did not suppress encoding of other traits. Judgments of likability were most strongly influenced by prosocial-antisocial behavior. Unlike prosocial and smart behavior, shyness influenced only adults' liking of the actor. Additional incongruent behaviors produced a substantial modification of impressions in all groups except for kindergartners who held positive impressions. (Contains 2 figures and 1 table.)