Day-to-day interactions often involve individuals interacting with groups, but little is known about the criteria that people use to decide which groups to approach or trust and which to avoid or distrust. Seven studies provide evidence for a "small = trustworthy" heuristic, such that people perceive numerically smaller groups as more benevolent in their character and intentions. As a result of this, individuals in trust-sensitive contexts are more likely to approach and engage with groups that are relatively small than those that are relatively large. We provide evidence for this notion across a range of contexts, including analyses of social categories (Studies 1 and 2), ad hoc collections of individuals (Study 3), interacting panels (Studies 4-6), and generalized, abstract judgments (Study 7). Findings suggest the existence of a general lay theory of group size that may influence how individuals interact with groups.