In order to ensure successful mating opportunities, it is critical that human perceivers accurately infer others' sexual interests. But how do perceivers achieve these inferences? For over 50 years, scientists have documented that the pupils dilate in response to sexual arousal. Despite the potential importance of this cue for mate selection, however, extant data have focused almost exclusively on the perspective of the individual experiencing arousal. Here, we demonstrate that outside observers exploit pupil dilation as a visible cue to others' sexual interests. We used reverse-correlation methods to derive facial images based on perceivers' mental representations of both state-based (sexually aroused, sexually unaroused) and trait-based (sexually promiscuous, sexually non promiscuous) markers of sexual interest. Next, we explored the phenotypic features that differentiated these faces, specifically the dilation of the pupils contained within each reverse-correlation image. Consistent with the notion that pupil dilation is a reliable cue to sexual arousal, sexually interested faces contained objectively larger and darker pupils than did sexually disinterested faces. Moreover, these differences were perceptually obvious to naive observers. Collectively, our results suggest that perceivers attend to an external cue - pupil dilation - when forming decisions about others' state-based and trait-based sexual interests. (C) 2016 Elsevier Inc. All rights reserved.