When less is more: like humans, chimpanzees (Pan troglodytes) misperceive food amounts based on plate size.

We investigated whether chimpanzees (Pan troglodytes) misperceived food portion sizes depending upon the context in which they were presented, something that often affects how much humans serve themselves and subsequently consume. Chimpanzees judged same-sized and smaller food portions to be larger in amount when presented on a small plate compared to an equal or larger food portion presented on a large plate and did so despite clearly being able to tell the difference in portions when plate size was identical. These results are consistent with data from the human literature in which people misperceive food portion sizes as a function of plate size. This misperception is attributed to the Delboeuf illusion which occurs when the size of a central item is misperceived on the basis of its surrounding context. These results demonstrate a cross-species shared visual misperception of portion size that affects choice behavior, here in a nonhuman species for which there is little experience with tests that involve choosing between food amounts on dinnerware. The biases resulting in this form of misperception of food portions appear to have a deep-rooted evolutionary history which we share with, at minimum, our closest living nonhuman relative, the chimpanzee.