Preference for symmetry in ornaments or faces in different species may have evolved because symmetry indicated mate quality, leading to advantages in natural selection. Alternatively, symmetry preference may reflect sensory biases that evolved because of the need for signal recognition. If so, selection for signal recognition may have led to preferences for any perceptual features which are easy to perceive, such as symmetry, figure-ground contrast, and surface continuity. Consequently, the general underlying mechanism would be perceptual fluency, i.e. the phenomenal experience of ease of perception. Consistent with this assumption, human participants preferred vertical symmetry to asymmetry, continuous to discontinuous surfaces, and high over low figure-ground contrast in pairs of random shapes without any biological significance. Moreover, the preferred features were objectively and subjectively easier to perceive.
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