People have characteristic ways of perceiving others’ personalities. When judging others on several traits, some perceivers tend to form globally positive and others tend to form globally negative impressions. These differences, often termed perceiver effects, have mostly been conceptualized as a static construct that taps perceivers’ personal stereotypes about the average other. Here, we assessed perceiver effects repeatedly in small groups of strangers who got to know each other over the course of 2–3 weeks and examined the degree to which positivity differences were stable versus developed systematically over time. Using second-order latent growth curve modeling, we tested whether initial positivity (i.e., random intercepts) could be explained by several personality variables and whether change (i.e., random slopes) could be explained by these personality variables and by perceivers’ social experiences within the group. Across three studies (ns = 439, 257, and 311), personality variables characterized by specific beliefs about others, such as agreeableness and narcissistic rivalry, were found to explain initial positivity but personality was not reliably linked to changes in positivity over time. Instead, feeling liked and, to a lesser extent, being liked by one’s peers partially explained changes in positivity. The results suggest that perceiver effects are best conceptualized as reflecting personal generalized stereotypes at an initial encounter but group-specific stereotypes that are fueled by social experiences as groups get acquainted. More generally, these findings suggest that perceiver effects might be a key variable to understanding reciprocal dynamics of small groups and interpersonal functioning.