Mass and density estimates contribute to perceived heaviness with weights that depend on the densities´ reliability

People perceive a smaller and denser object to be heavier than a larger, less dense object of the same mass. We developed a new model of heaviness perception that can explain this size-weight illusion. Modeling followed recent insights on principles of information integration. Perceived heaviness is modeled as a weighted average of one heaviness estimate derived from object mass and another one derived from object density with weights that follow estimate reliabilities. In an experiment, participants judged the heaviness of 18 objects using magnitude estimation methods. Objects varied in mass and density. We also varied the reliability of density information by varying visual reliability: Participants were blindfolded or had strongly impaired, mildly impaired or full vision. Because participants lifted each object via a string they required visual information on object size to assess object density. The pattern of mass and density influences on judged heaviness confirmed model predictions. Also as predicted, density influences on judged heaviness increased with increasing reliability, whereas mass influences decreased. Individual and average data were well fit by the model (r²s > 0.96). Density information contributed for 14%, 21 % and 29% to heaviness, when vision was strongly impaired, mildly impaired or not impaired, respectively. Overall, the results highly corroborate our model, which appears to be promising as unifying framework for a number of findings on the size-weight illusion.