The face size illusion is specific to human faces

The faces of conspecifics are a critical feature of our social world. The visual system includes specialised processes for perceiving upright faces, which are not engaged to the same extent when faces are inverted (the face inversion effect). Recently, a face size illusion has been described in which upright faces are perceived as physically smaller than identical inverted faces. This effect appears highly specific to faces and does not occur for other stimuli, such as objects, hands, bodies and letters. We investigated whether this face size illusion is specific to faces in general or to human faces in particular. On each trial, participants saw two faces, one upright and one inverted and judged which face appeared physically larger. Across blocks, participants saw faces of humans, monkeys and cats. For human faces, there was a clear bias for upright faces to be perceived as smaller than inverted ones, consistent with previous findings. No such effect was found for the faces of cats or monkeys, for which there was a significant bias in the opposite direction. These results provide further evidence for the specificity of the face size illusion, showing that it is specific not just to faces, but to human faces.