Weird illusion in which adding weight to something makes it feel lighter

"Impossible Somatosensation" is a paper written by Isabel Won, Steven Gross, and Chaz Firestone Johns of Hopkins University. From the abstract: "We show that, even in 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. Impossibility can not only be seen, but also felt."

Stimuli and Procedure

Subjects saw three opaque boxes in a stack, which we refer to here as Boxes A, B, and C. Subjects were instructed to perform two lifts, one immediately after the other: In one case, they lifted Boxes A, B, and C together; in another case, they lifted Box A alone. Here in Experiment 1, subjects lifted the boxes simply by grasping them with their hands, in whatever posture felt natural (though later experiments varied this grasp posture).

After the two lifts (whose order was counterbalanced across subjects), subjects were asked which lift felt heavier (or, for half of the subjects, which lift felt lighter), and the experimenter recorded the subjects response.

Results

Subjects overwhelmingly reported that Box A alone felt heavier than Boxes A, B, and C together (90% of subjects reporting A heavier than A+B+C, binomial probability test, p<.001 against chance [50%] responding; Figure 3)1. However, this result should be impossible, because the sum of weights over a set of objects could never be less than the sum of weights over a subset of those objects: Unless the boxes somehow changed between lifts, Box A couldnt weigh more than a group of weighted objects that includes Box A as a member.