It is well known that people overestimate the orientation of both geographical and man-made sloped surfaces by between 5°-20°. More recently, work has shown that when people are encumbered by wearing a heavy backpack they overestimate hills and distances even more than a group not wearing heavy backpacks; however, the backpack manipulation has since been shown to be a demand effect-that is, being encumbered does not affect perception-it only biases those people influenced by it to give estimates the experimenters are seeking. Here we first show that expended effort and intent have never actually been manipulated between subjects in any of the work on steepness estimates, and expended effort has never been explicitly manipulated between subjects for distance estimates. When they are, they do not affect steepness or distance estimates. Our work is the first to experimentally manipulate effort and intent, to do this between subjects, while controlling for demand characteristics and checking to see whether the effort manipulation was effective. The experimental manipulation of effort and especially intent may be of benefit to parsing out perceptual effects from more cognitive or postperceptual processes in future work. Public Significance StatementThis study investigated whether our perception of slant and distance are infiltrated by top-down influences or encapsulated from them. We directly manipulated effort (being pushed or pushing themselves) and intent (whether they thought they would go up the hill or not) using a wheelchair. We found no differences in steepness or distance estimates between effort or intent conditions. This work is the first to manipulate effort and intent directly between subjects, while controlling for experimental demand, and shows perception of steepness and distance are stable in the face of various physical and cognitive states of the observer.

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Previous work has shown that people overestimate their own body tilt by a factor of about 1.5, the same factor by which people overestimate geographical and man-made slopes. In Experiment 1 we investigated whether people can accurately identify their own and others' tipping points (TPs)the point at which they are tilted backward and would no longer be able to return to upright as well as their own and others' center of mass (COM)the relative position of which is used to determine actual TP. We found that people overestimate their own and others' TP when tilted backward, estimate their own and others' COM higher than actual, and that COM estimation is unrelated to TP. In Experiment 2, we investigated people's intuitive beliefs about the TP. We also investigated the relationship between phenomenal TP and perceived vertical. Whether verbally (conceptually) estimating the TP, drawing the TP, or demonstrating the position of the TP, people believe that the TP is close to 45°. In Experiment 3, we found that anchoring influences phenomenal TP and vertical. When accounting for starting position, the TP seems to be best predicted by an intuitive belief that it is close to 45°. In Experiment 4, we show that there is no difference in phenomenal TP and vertical when being tilted about the feet or waist/hips. We discuss the findings in terms of action-perception differences found in other domains and practical implications.