So I take walks in this park where there are narrow curbs, not much wider than the width of my shoes. Nevertheless, I can walk on these curbs for blocks without falling off, placing one foot in front of the other. Sometimes I need to balance myself with my arms, but I never fall off the curb.
I swear, however, that if I had to walk on a beam even twice as wide for fifty feet between two fifty story buildings, I would most likely fall.
What is going on here?
You’d be scared shitless if you were walking on a 12"-wide beam for 50 feet. It’s also windy up there. Just my WAG.
I’ve wondered the same thing. And I doubt it’s a matter of fear. I can walk along a curb without a problem, but stick me on a balance beam three inches off the ground, and it will take a lot more effort on my part. The only thing I can think of is that when you misplace your foot on a curb the fraction of an inch that it takes to throw you off balance, you don’t realize it because there’s other surface to take up the slack. In other words, it doesn’t take a lot to lose your balance, and when you’re walking on a curb, you can’t see that you’ve made that miniscule misstep.
Ignore me; I misunderstood. My point was about balancing along the edge of a curb that is actually adjoined by a sidewalk or something. I seem to be able to walk along one of those with no problem, maintaining a straight and narrow path, but can balance on a beam to save my life.
Nothing scientific, but I think it’s worth noting that in all real world scenarios the the balance beam is going to be relatively much less stable than a concrete curb. Could be a big factor, could be a small one, tough to say.
My assumption is that it’s 80% mental, you’re thinking about falling and are therefore more apt to do so. Personally I can walk for miles on a 4" beam or curb equally well, so I take that to imply that there’s a specific personal trait that causes it, which mean’s it’s probably all in your head.
Fascinating question, one that I’ve also asked myself. I recall reading that balancing has a lot to do with being relaxed, and it’s a lot harder to relax 50 feet off the ground. I’ve also noticed this phenomenem when snowboarding, where worrying about falling seems to cause me to fall.
What’s going on? Well, I think a lot of what’s going on is this bit “I swear, however, that if I had to walk on a beam even twice as wide for fifty feet between two fifty story buildings, I would most likely fall.”
This is just a wild guess from your side. 
Other than that I can think of two important things. One of them mentioned already.
Balancing requires you to be somewhat relaxed. If you almost stumble on the curb, you might easily regain your balance. If you almost stumble 400 feet in the air, you might overcompensate and fall to your death.
Do you really never fall off the curb? I suspect there’s a bit of psychological trick of the mind involved as well. Putting your foot down wrong once in a long while balancing on the curb is pretty much a non-event. Stepping wrong on that beam on the other hand…
I remember reading/hearing that is also has to do with your forward momentum. Sort of like when riding a bike, it’s easy to balance when you’re moving forward, but more difficult when moving slowly or not at all.
So when you’re purposely trying to balance, and taking it slowly, that could be the problem in itself. Walking along a curb (I like walking on belgium blocks myself), you maintain your walking speed and don’t look straight down because you’re not worried, and subconsciously you compensate with your next step if you’re off a little.
I’ve tried it… walking slowly and looking at my feet vs. just normal walking while looking straight ahead. I do much better with the latter.
I don’t claim any of this to be fact, I’m sorry if it’s wrong. I figured I’d at least post so that others can say that’s either right or wrong.
Fear and nervousness. In another life, about 10 years ago, I was a framer. Where I worked we did mostly hog barns. We would walk along the top of a 6" wide wall that was about 12’ above a concrete floor (8’ studded wall on top of a 4’ concrete beam) when we were doing rafters or whatever. We would even do it backwards when marking the rafter spacing. Bump that up another 38’ and I probably would have fallen off, but at 12’ I was fine.
I remember one of my first summer jobs was a “corn puller”
Simply meant that the corn would come into large bins and my job was to pull the doors open evenly so that those who chopped and boxed the corn had an even supply. Paid all of 75 cents an hour back in 1961. A summer college job for me to make a few backs for the next year at FSU.
Had to run along a 4 by 4 directly above the knives that the corn boxers used.
And I mean RUN in order to keep the line going. I only lasted one day—was way too slow. I was 18 years old, but really couldn’t do it with any speed —
—but the 14 and 15 year olds that I worked with could actually run super fast along those narrow boards and never miss a step or fall into the knives.
I never in my life have felt so old at the age of 18. Thank Gawd for child labor laws these days.