Human observers extract perceptual summaries for sets of items after brief visual exposure, accurately judging the average size of geometric shapes (Ariely, 2001), walking direction of a crowd (Sweeny, Haroz, & Whitney, 2013), and the eye gaze of groups of faces (Sweeny & Whitney, 2014). In addition to such actuarial summaries, we hypothesize that observers also extract social information about groups that may influence downstream judgments and behavior. In four studies, we first show that humans quickly and accurately perceive the sex ratio of a group after only 500 ms of visual exposure. We then test whether these percepts bias judgments about the group's social attitudes and affect the perceiver's sense of belonging. As the ratio of men to women increased, both male and female perceivers judged the group to harbor more sexist norms, and judgments of belonging changed concomitantly, albeit in opposite directions for men and women. Thus, observers judge a group's sex ratio from a mere glimpse and use it to infer social attitudes and interpersonal affordances. We discuss the implication of these findings for a heretofore overlooked hurtle facing women in male-dominated fields (e.g., science, technology, engineering, or mathematics): how the ratio of men to women provides an early visible cue that signals an individual's potential fit. (PsycINFO Database Record (c) 2018 APA, all rights reserved).