Who is being deceived? The experimental demands of wearing a backpack

A growing literature argues that wearing a heavy backpack makes slopes look steeper and distances seem longer (e.g., Proffitt, 2006). To test for effects of experimental demand characteristics in a backpack experiment, we manipulated the experimental demand of the backpack and then used a postexperiment questionnaire to assess participants' beliefs about the purpose of the backpack. For participants in the low-demand condition, an elaborate deception was used to provide an alternative explanation of the requirement to wear a heavy backpack (i.e., that it held EMG equipment). The highest slope judgments were found for those undeceived participants who guessed that the backpack was intended to affect their slope perception and also reported that they thought they were affected by it. When persuaded that the backpack served another purpose, participants' slope estimates were no different from those of participants not wearing a backpack. These findings suggest that backpack effects, and other reported effects of effort on perception, are judgmental biases that result from the social, not physical, demands of the experimental context.