Charpentier illusion

The Charpentier Illusion (a.k.a. the Size-Weight Illusion) was first formally demonstrated in 1891 (ref.)

Experimental subjects are shown two objects of differing sizes (but which have been carefully designed to have identical weights). Most people guess that the larger object might be heavier, but, when asked to pick them up they consistently judge the smaller object to be heavier than the larger one.

When we lift two differently-sized but equally-weighted objects, we expect the larger to be heavier, but the smaller feels heavier. However, traditional Bayesian approaches with “larger is heavier” priors predict the smaller object should feel lighter; this Size-Weight Illusion (SWI) has thus been labeled “anti-Bayesian” and has stymied psychologists for generations".

Source :PeerJ. 2016; 4: e2124

The illusion still doesn't have an accepted explanation, and is now further complicated by experiments which have shown that subjects will often rate the weight of an object as being heavier than a group of objects which include the first object - even when they are well aware that this result is clearly impossible.

We show that, even under full-cue conditions with objects that can be freely inspected, subjects can be made to experience a single object alone as feeling heavier than a group of objects that includes the single object as a member—an impossible and phenomenologically striking experience of weight."

Source : PsyArXiv Preprints, 2021