You might think humans are equally good at estimating distances no matter which direction they’re looking. After all, we use the same visual tools to make those estimates — binocular disparity (the different views we see from each eye), occlusion (whether one object is in front of or behind another), and so on. But consider the situation depicted to the right. Nora is inching her way down a steep rock column, with near-vertical drops on either side of her. If she underestimates the distance to flat ground below, she might decide she doesn’t need to worry about falling. Overestimating the vertical distance isn’t as big a problem: if she descends too slowly and carefully, she’ll still live to tell the story.
On flat ground, overestimating distances could spell trouble: you might pack too much food for a hike, unnecessarily burdening yourself and perhaps not even making it to your destination. In fact, people do make systematic errors in estimating distance based on how much weight they’re carrying; it’s possible that they might make the same sort of errors estimating vertical distance. Since it takes more effort to climb up a mountain than climbing down, maybe we misjudge up distances as longer than down distances.
Russell Jackson and Lawrence Cormack took college students to the base of a 40-foot-tall wall and asked them to estimate the distance to the top by telling a research assistant to back away from the wall until their distance from the wall was equal to its height. Then they took them to the top of the wall and asked them to estimate the distance down using the same system (actually, half the students estimated the distance down first). This graph shows the results:
If the students had seen the distance as the same in both directions, their responses would have fallen along the diagonal line. If they accurately estimated distances, the responses would have fallen where the three lines intersect. In fact, nearly all respondents overestimated both the distance up and down the wall. Looking closer at the graph, you can see that the estimates for the distance down were nearly always larger than for the distance up. The average estimates for down distance were significantly larger than for up distance.
Jackson and Cormack argue that this overestimation of down-distance is beneficial from an evolutionary perspective. If we think a potential fall is worse than it really is, we’re probably going to be more careful when we’re next to a precipitous drop-off. This danger-avoidance adaptation trumps any overestimation we might make based on the amount of work it might take to climb up a wall. Playing it safe, it seems, is so important that it’s worthwhile for the perceptual system to make systematic errors in judging distance.
Jackson, R.E., Cormack, L.K. (2007). Evolved navigation theory and the descent illusion. Perception & Psychophysics, 69(3), 353-362.