Evolved navigation theory and the plateau illusion

Most people anecdotally feel that the distance extending toward a cliff or slope appears shorter than the same distance extending away from it. This odd impression persists, despite the distance being equal across both conditions and humans encountering such a scenario daily in the navigation of stairs, slopes, curbs, and vertical surfaces protected by handrails. We tested three sets of competing predictions about this previously uninvestigated phenomenon. Data from two experiments coincided with the well-established predictions from evolved navigation theory. Contrary to anecdotal expectations, observers perceive the distance extending toward the edge of a steep slope to be longer than the distance extending away from it. We title this the plateau illusion and suggest that it may be an embodied process that arose over evolutionary time in response to navigation risks.