Why do most faces look thinner upside down?

Faces are found generally to be perceived as thinner when viewed upside down. When a face is viewed upright, the internal features are thought to influence the perception of face shape. However, when inverted, it has been proposed that disruption to holistic processing means that these factors can no longer be used to judge the shape of a face. We show that it is not the case that an inverted face reverts to some average shape whereby fat faces appear thinner upside down whereas thin faces appear fatter. The fact that the illusion appears to occur for most face shapes is discussed with regard to the horizontal–vertical illusion.