Evolved navigation theory and the environmental vertical illusion

This study outlines a previously unknown, large illusory component to one of the most common psychological experiences. Evolved navigation theory (ENT) suggests that perceptual and navigational mechanisms reflect navigational costs over evolution. Vertical surfaces pose a distinct cost of falling not present in horizontal navigation. However, horizontal surfaces sometimes form retinally vertical images and researchers often assume that retinal image determines distance perception. We tested ENT-derived predictions suggesting that observers would overestimate surface lengths based on environmental, not retinal, verticality. Participants drastically overestimated environmentally vertical surfaces only and did so at a magnitude related to surface length. These results replicate across multiple settings and methods and are supported by previous studies. Although researchers often assume that selection pushes perceptual mechanisms toward objective accuracy, this study suggests that genetic fitness can sometimes benefit from systematic illusions.