A ubiquitous illusion of volume: Are impressions of 3D volume captured by an ‘Additive Heuristic’?

Several empirical approaches have attempted to explain perception of 2D and 3D size. While these approaches have documented interesting perceptual effects, they fail to offer a compelling, general explanation of everyday size perception. Here, we offer one. Building on prior work documenting an ‘Additive Area Heuristic’ by which observers estimate perceived area by summing objects’ dimensions, we show that this same principle — an ‘additive heuristic’ — explains impressions of 3D volume. Observers consistently discriminate sets that vary in ‘additive volume’, even when there is no true difference; they also sometimes fail to discriminate sets that truly differ (even by amounts as much as 200%) when they are equated in ‘additive volume’. However, this heuristic also has limits: when volume varies to large extents, observers may rely on both ‘additive’ and true, mathematical volume to make volume judgments. These results suggest a failure to properly integrate multiple spatial dimensions, and frequent reliance on a perceptual heuristic instead.