I’m usually disappointed when I try to take a picture of a steep precipice—it never seems as impressive in the photo as it did when I was standing right there. Take this photo, for example. It’s a nice shot of my daughter Nora, taken on our hike in the Great Smoky Mountains this past summer, but you just don’t get much sense of the dizzying precipice she’s standing on the edge of.
Later that summer, on Lake Powell, Utah, I finally managed to get a shot that conveyed some drama:
But even this shot doesn’t really show the paralyzing terror Nora felt as she peered over the 100-foot drop-off to her left (okay, it was probably more my terror as her father, but you get the idea).
One reason for this is that humans routinely overestimate the steepness of slopes when they’re actually standing on them—guessing that a 10° pitch is actually 30°. Something about physically dealing with the environment exaggerates our perception of its characteristics. On our weekend hike the Smokeys, laden with backpacks, it sure felt like we had hiked a lot more than three miles each way! But is that just the exhaustion talking, or is our perception really affected by the burdens we carry?
A group led by Dennis Proffitt conducted a clever set of experiments to answer that question. In their first experiment, they asked participants to make repeated judgments about the distance a construction cone placed from 2 to 14 meters away. Though they weren’t allowed to walk to the cone, participants were given a ruler to give a frame of reference. The trick was, half the participants wore backpacks laden with one-fifth of their body weight. Here are the results:
People wearing backpacks consistently estimated the distances as longer than people not wearing packs. But maybe these results could be explained by the fact that participants were wearing backpacks. Even though they weren’t required to actually walk the distance, they may have gotten some clues as to what the experiment was about when the researchers asked them to guess distances while wearing backpacks.
So Proffitt’s team devised a subtler way of testing the same thing. A few years earlier, Frank Durgin had led a group doing research on a treadmills. If volunteers were shown a virtual-reality display indicating they were actually moving while walking on a treadmill, then afterwards, off the treadmill, they were able to march in place (wearing a blindfold) better than another group who had watched only a still image on the treadmill. The people who hadn’t been exposed to motion on the treadmill marched forward a bit when told to march in place, as if to compensate for the lack of motion on the treadmill.
Proffitt’s group replicated Durgin’s work, but then added another dimension to the study. Both before and after the treadmill exercise, participants were led blindfolded to a hallway where they performed the same distance-judgment task that the backpack group had done earlier. Those who had seen a virtual reality display indicating they were moving estimated the distance to be about ten percent shorter than they had before the treadmill exercise. Those who had only seemed to be walking in place estimated the distance to be about ten percent longer than they had before. Proffitt et al. argue that the non-moving group felt like they needed to expend more effort to go the same distance—because all the exertion on the treadmill had “gotten” them nowhere. The group that saw the VR motion, by contrast, was amply rewarded for its work, and so felt that the distance seemed shorter than it had before.
Proffitt, D. R., Stefanucci, J., Banton, T., & Epstein, W. (2003). The role of effort in perceiving distance. Psychological Science, 14(2), 106-112.