Box shape influences the size-weight illusion during individual and team lifting

Objective: The effects of box shape-specifically width and height-on the perception of heaviness were evaluated during individual and team lifting. Background: Large objects are perceived to be as much as 50% lighter than smaller objects with the same mass. This size-weight illusion presents an obvious risk when lifting large and heavy boxes. Recent research has shown that shape influences this illusion. Specifically, increases in length and width do not produce identical decreases in perceived heaviness. However, this effect has been documented only in individual lifting, mostly with small objects. Method: Individuals and teams lifted large boxes and reported their perceptions of heaviness. The mass, height, and width of the boxes were varied independently to determine their unique effects on perceived heaviness. Results: For both types of lift, increasing width produced a greater mean illusory drop (expressed as a percentage decrease with 95% confidence intervals) in perceived heaviness (24 ± 7% during individual lifting and 41 ± 8% during team lifting) than increasing height (15 ± 7% during individual lifting and 18 ± 8% during team lifting). Conclusion: Size and shape are important factors in perceiving the heaviness of boxes during both individual and team lifting. Application: To avoid misperceiving weight and risking injury, lifters should be careful when approaching larger (especially wider) boxes.