Weight perception and the haptic size–weight illusion are functions of the inertia tensor.

TLDR

Weight perception, including the size‑weight illusion, was hypothesized to be governed by invariants of rotational dynamics. In Experiments 1–6 the authors varied the rotational inertia of occluded objects independently of mass, size, and torque, simulating specific inertia‑tensor eigenvalue patterns while keeping mass, volume, and visible size constant. Perceived heaviness depended solely on rotational inertia, with size effects modulated by inertia‑tensor eigenvalue patterns—heaviness decreased or increased across sets with constant mass/increasing volume or increasing mass/constant volume—demonstrating that weight perception and the size‑weight illusion rely on stimulus invariants rather than inference.

Abstract

The complex effects of mass and volume on weight perception (e.g., the size-weight illusion) were hypothesized to follow simply from invariants of rotational dynamics. In Experiments 1-3, the rotational inertia of wielded, occluded objects was varied independently of mass, size, and torque. Perceived heaviness depended only on rotational intertia. Reanalysis of J. C. Stevens and L. L. Rubin's (1970) study revealed that size's influence on weight perception depends on specific patterns of the eigenvalues of the inertia tensor. These patterns were simulated in Experiments 4-6 with objects of fixed mass, volume, and visible size. Perceived heaviness decreased and increased, respectively, over object sets with the eigenvalue patterns of (a) constant mass, increasing volume and (b) increasing mass, constant volume. Weight perception and the size-weight illusion depend on stimulus invariants, not inference.