We quantitatively examined the difference in perceived size between upright and inverted faces using the method of constant stimuli. The stimuli included eight face images modified from two cartoon faces produced by Kitaoka (2007, http://www.psy.ritsumei.ac.jp/∼akitaoka/kao-e.html and 2008, Cognitive Psychology 5 177–185) and six photographic faces, including a photographic face used by Thompson (2010, http://illusioncontest.neuralcorrelate.com/2010/the-fat-face-thin-fft-illusion/). Experiment 1 showed that an upright face and outline were perceived to be significantly smaller than an inverted face and outline, respectively. Moreover, the amount of the size underestimation in the face stimulus condition was significantly larger than that in the outline stimulus condition. Experiment 2 showed that an upright face was perceived to be significantly smaller than 90° and 270° rotated faces, whereas an inverted face was not perceived to be significantly larger than a 90° or 270° rotated face. Experiment 3 showed that upright faces were perceived to be significantly smaller than upright and inverted outlines, whereas inverted faces were not perceived to be significantly larger than upright or inverted outlines. Experiments 4 and 5 showed that upright photographic faces were also perceived to be significantly smaller than inverted photographic faces. These results provide quantitative evidence for a size underestimation of upright faces.
