Size–weight illusion

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The size–weight illusion, also known as the Charpentier illusion, is named after the French physician Augustin Charpentier[1] because he was the first to demonstrate the illusion experimentally.[2][3][4] It is also called De Moor's illusion, named after Belgian physician Jean Demoor (1867–1941).[5]


The illusion occurs when a person underestimates the weight of a larger object (e.g. a box) when compared to a smaller object of the same mass. The illusion also occurs when the objects are not lifted against gravity, but accelerated horizontally, so it should be called a size-mass illusion.[6] Similar illusions occurs with differences in material and colour: metal containers feel lighter than wooden containers of the same size and mass,[7] and darker objects feel heavier than brighter objects of the same size and mass.[8][9] These illusions have all been described as contrast with the expected weight,[10] although the size-weight illusion occurs independent of visual estimates of the volume of material[11] and the illusion does not depend on expectations, but occurs also if visual size information is only provided while already lifting.[12] The expected weight or density can be measured by matching visible and hidden weights, lifted in the same manner. This gives an expected density of about 1.7 for metal canisters and 0.14 for polystyrene blocks.[13] Density expectations may assist in selecting suitable objects to throw.[14]


An early explanation of these illusions was that people judge the weight of an object from its appearance and then lift it with a pre-determined force. They expect a larger object to be heavier and therefore lift it with greater force: the larger object is then lifted more easily than the smaller one, causing it to be perceived as lighter.[15] This hypothesis was disproved by an experiment in which two objects of the same mass, same cross section, but different height were placed on observers' supported hands, and produced a passive size–weight illusion.[16] Recent studies have also shown that the lifting force quickly adapts to the true mass of the objects, but the size–weight illusion remains.[17][18][19] The illusion therefore cannot be explained by the manner of lifting, and must be due to some perceptual rescaling based on prior expectations. The rescaling has been described as sub-optimal (anti-Bayesian), in that the central nervous system integrates prior expectations with current proprioceptive information in a way that emphasises the unexpected information rather than taking an average of all information.[19][20] It has also recently been suggested that the illusion may not be anti-Bayesian, but may instead rely on more complex yet still optimal inference processes than traditionally suggested.[21]

Other models describe the rescaling as partly beneficial, in that it enhances discrimination. Contrast effects are common in many perceptual modalities, and are similar to physiological adaptation. Adaptation can be explained as a change in the gain of the system, the gain being set to the appropriate level for maximum discrimination and for protection against sensory overload. Contrast effects may similarly be related to efficient neural coding.[19] If the selected range is either too high or too low, as in the size–weight illusion, there is both a contrast illusion and a loss of discrimination. It has been found that weight discrimination deteriorates if objects are lighter than their expected density,[22][23] or heavier than their expected density.[22] Models of this type can account for perceptual rescaling without involving the manner of lifting.

It has also been demonstrated that, taking three empty matchboxes, and putting a weight in one of them, the weighted box lifted on its own feels heavier than all three boxes lifted together with the heavy one on top.[24][25][26]

See also[edit]


  1. ^ Charpentier, A (1891). "Analyse experimentale: De quelques elements de la sensation de poids [Experimental study of some aspects of weight perception]". Arch Physiol Norm Pathol. 3: 122–135.
  2. ^ Koseleff, P (1957). "Studies in the perception of heaviness". Acta Psychol. 13: 242–252. doi:10.1016/0001-6918(57)90023-9.
  3. ^ Murray, D J; Ellis, R R; Bandomir, C A; Ross, H E (1999). "Charpentier (1891) on the size-weight illusion". Percept Psychophys. 61 (8): 1681–1685. doi:10.3758/bf03213127. PMID 10598479.
  4. ^ Nicolas, S; Ross, H E; Murray, D J (2012). "Charpentier's papers of 1886 and 1891 on weight perception and the size-weight illusion". Perceptual and Motor Skills. 115 (1): 1–23. doi:10.2466/24.22.27.PMS.115.4.120-141. PMID 23033750. S2CID 35509359.
  5. ^ Huang, I. (1 July 1945). "The Size-Weight Illusion in Relation to the Perceptual Constancies". The Journal of General Psychology. 33 (1): 43–63. doi:10.1080/00221309.1945.10544494. ISSN 0022-1309.
  6. ^ Plaisier, Myrthe A.; Smeets, Jeroen B. J. (2012-08-09). Paul, Friedemann (ed.). "Mass Is All That Matters in the Size–Weight Illusion". PLOS ONE. 7 (8): e42518. Bibcode:2012PLoSO...742518P. doi:10.1371/journal.pone.0042518. ISSN 1932-6203. PMC 3415412. PMID 22912704.
  7. ^ Seashore, C E (1899). "Some psychological statistics.2. The material weight illusion". Univ Iowa Stud Psychol. 2: 36–46.
  8. ^ De Camp, J E (1917). "The influence of color on apparent weight: a preliminary study". J Exp Psychol. 2 (5): 347–370. doi:10.1037/h0075903.
  9. ^ Payne Jr., M. Carr (1958). "Apparent weight as a function of color". The American Journal of Psychology. 71 (4): 725–730. doi:10.2307/1420330. JSTOR 1420330. PMID 13627281.
  10. ^ Jones, LA (1986). "Perception of force and weight: theory and research". Psychol Bull. 100 (1): 29–42. doi:10.1037/0033-2909.100.1.29. PMID 2942958.
  11. ^ Plaisier, Myrthe A.; Smeets, Jeroen B.J. (November 2016). "Object size can influence perceived weight independent of visual estimates of the volume of material". Scientific Reports. 5 (1): 17719. doi:10.1038/srep17719. ISSN 2045-2322. PMC 4667212. PMID 26626051.
  12. ^ Plaisier, Myrthe A.; Kuling, Irene A.; Brenner, Eli; Smeets, Jeroen B. J. (June 2019). "When Does One Decide How Heavy an Object Feels While Picking It Up?". Psychological Science. 30 (6): 822–829. doi:10.1177/0956797619837981. ISSN 0956-7976. PMC 6560521. PMID 30917092.
  13. ^ Ross, H.E. (1969). "When is a weight not illusory?". Q J Exp Psychol. 21 (4): 346–355. doi:10.1080/14640746908400230. PMID 5378275. S2CID 41020343.
  14. ^ Zhu, Q; Bingham, G (2010). "Learning to perceive the affordance for long-distance throwing: Smart mechanism or function learning?". J Exp Psychol Hum Percept Perform. 36 (4): 862–875. CiteSeerX doi:10.1037/a0018738. PMID 20695705.
  15. ^ Müller GE, Schumann F. Über die psychologischen Grundlagen der Vergleichung gehobener Gewichte. Pflügers Arch XLV: 37–112, 1889.
  16. ^ Usnadze, D (1931). "Über die Gewichtstauschung und ihre Analoga [Aspects of weight illusions]". Psychol Forsch. 14: 366–379. doi:10.1007/bf00403879. S2CID 144645843.
  17. ^ Mon-Williams, M; Murray, A H (2000). "The size of the visual size cue used for programming manipulative forces during precision grip". Exp Brain Res. 135 (3): 405–410. doi:10.1007/s002210000538. PMID 11146818. S2CID 23921941.
  18. ^ Flanagan, J R; Beltzner, M A (2000). "Independence of perceptual and sensorimotor predictions in the size-weight illusion". Nature Neuroscience. 3 (7): 737–741. doi:10.1038/76701. PMID 10862708. S2CID 205096833.
  19. ^ a b c Brayanov, J; Smith (2010). "Anti-Bayesian" biases in sensory integration for action and perception in the size–weight illusion". J Neurophysiol. 103 (3): 1518–1531. doi:10.1152/jn.00814.2009. PMC 4422348. PMID 20089821.
  20. ^ Ernst, M O (2009). "Perceptual learning: inverting the size-weight illusion". Curr Biol. 19 (1): R23–R25. doi:10.1016/j.cub.2008.10.039. PMID 19138585.
  21. ^ Peters, Megan A.K.; Ma, Wei Ji; Shams, Ladan (2016-06-16). "The Size-Weight Illusion is not anti-Bayesian after all: a unifying Bayesian account". PeerJ. 4: e2124. doi:10.7717/peerj.2124. ISSN 2167-8359. PMC 4918219. PMID 27350899.
  22. ^ a b Ross, H E; Gregory, R L (1970). "Weight illusions and weight discrimination - a revised hypothesis". Q J Exp Psychol. 22 (2): 318–328. doi:10.1080/00335557043000267. PMID 5431405. S2CID 41129834.
  23. ^ Jones, L F; Burgess, P R (1998). "Neural gain changes subserving perceptual acuity". Somatosensory Motor Res. 15 (3): 190–199. doi:10.1080/08990229870754. PMID 9874518.
  24. ^ "Three Card Box Illusion". YouTube. Archived from the original on 2021-12-19.
  25. ^ Koseleff, Paul (December 1937). "Eine Modifikation des "Charpentier-Effektes"". Psychologische Forschung. 21 (1): 142–145. doi:10.1007/BF02441205. S2CID 144221181.
  26. ^ Firestone, Chaz; Gross, Steven; Won, Isabel. "Impossible Somatosensation". Retrieved 2019-06-06.