Turning a molehill into a mountain is easy. All you need to do is go for a long run or bike ride. The further you run the taller the hill grows, until it becomes so steep as to appear impossible to climb. But even when you perceive that hill as a mountain, your feet still correctly respond to the tiny molehill.

People consistently overestimate how steep hills are – an interesting perceptual problem studied by Dennis Proffitt and his colleagues. Here’s the first cool finding: Hills appear steeper from the top than from the bottom. Although people generally overestimate how steep hills are (often by 20 to 30 degrees!), they make larger errors when looking down. Proffitt, Bhala, Gossweiler, and Midgett (1995) argued that this reflects the effort and risks associated with climbing and descending. Going down a hill is simply riskier than climbing up. You can walk up a 30-degree hill (although it isn’t much fun), but walking down that slope just isn’t safe.

Even toddlers can see the difference in steepness and risk from the top and bottom. Adolph, Eppler, and Gibson (1993) found that many toddlers would try to walk up all sorts of slopes – even when the slopes were so steep (30 to 40 degrees) that the toddlers ended up falling. But when those same toddlers were at the top and looking down, they were much more cautious. Even with low slopes that they had walked up successfully, they would sit and find a way to slide down. Smart kids. Early in development as toddlers are learning to guide their walking, slopes appear riskier from the top.

Proffitt and colleagues suggested that your perception of the hill reflects how much effort will be required to traverse it. From the bottom, the effort to climb is less that the effort to descend from the top. Your perception of steepness is a combination of how steep the hill really is plus an indication of how much work the hill will take. Perception is telling you just how hard and risky that climb or descent is.

Proffitt and his colleagues tested this idea by making the hill feel like it would be harder to climb. In a clever experiment, they asked people to judge how steep a hill was and then sent the people out for a run. They asked the participants to choose a length that would be challenging and told them where to meet immediately after the run. After the run, Proffitt and colleagues asked the exhausted runners to judge how steep a different hill was. For both shallow (5 degrees) and steep (30 degrees) hills, people saw the hills as steeper after the run when they were tired. In other words, taking a long run turned even the 5-degree molehill into a 25-degree mountain! As a regular bicycle commuter, I am intimately familiar with this effect. Hills look really ugly at the end of a long ride.

In other research, Proffitt and his colleagues have looked at how a variety of manipulations change the effort required to climb a hill and thus the perception of slope. For example, hills appear steeper when people wear a heavy backpack. In addition, Bhalla and Proffitt (1999) found that both physical fitness and age affect these judgments. People who are out-of-shape see the hills as more steep than people who are fit. And those molehills turn into mountains as we age as well. Proffitt and his colleagues have clearly demonstrated the slope perception is guided by affordances, that is the action potential of the individual (see my earlier post on desire making objects appear closer for another example of action potential biasing perception). 

Two different methods of judging hill steepness

But here is the interesting and surprising past: If people misperceive how steep hills are, why don’t they constantly fall down? We look at the molehill and think it is a 25-degree mountain in front of us instead of the 5-degrees it really is. We should lift our feet high enough for climbing a 25-degree slope and stumble as we find the ground further below our feet than we anticipated. We should be constantly stumbling and tumbling our way up and down hills. But we don’t. Generally we climb hills without falling.

Through their explorations of this interesting conundrum, Proffitt and his colleagues have argued that there two distinct perceptual systems. One systems leads to conscious awareness and the other directly guides action. The estimates of hill slant reflect the conscious awareness pathway. Proffitt has people name the slant and visually display the slant with an adjustable disk (see part A of the picture). Both the verbal and visual measures lead to large and consistent overestimates, particularly if you are tired, carrying that heavy backpack, or out of shape.

But Proffitt also had people make a behavioral judgment, using the tilt board in part B of the picture. To do this, people place their hands on the tilt board and match the board to the hill slant, without looking. When using this behavioral judgment, people more correctly assess the slope. The perceptual system that guides action gets the slope right. Even when you are tired and the molehill looks like a mountain, your feet know the truth.

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