Is fear of falling (from great heights) instinctual or
learned?
There is some evidence that the fear of falling is instinctual, though there is also some that points the other way. To my (limited) knowledge, no one knows for sure. The best example of evidence for it being an inborn fear is an experiment where infants who were at the crawling stage were prompted to crawl towards their mothers across a floor. This floor had a glass section where there was a two-foot drop under it. The infants would all stop at the edge of the drop, despite the fact that they could continue on the glass floor. I think these kids were all about 6-7 months old, so critiques say they may have learned the fear in that time.
Someone else will have to tell you more than that 
I’ve seen the same experiment as mnemosyne has. Note that at earlier ages the babies don’t recognize the drop and keep going across the glass.
One would think a fear of falling would be an important survival trait that would have been naturally selected into our psychological makeup (the others having fallen off a cliff or something before they could breed) but these things sometimes have a way of not being logical.
Wow. The boards are so slow my response from an hour ago was sucked into nowhere. Here it is again:
From my Sensation & Perception book (Coren & Ward, 1979):
A cute slide show can be found here
It’s not conclusive, but if you ask me, it’s innate.
I’d say it’s learned, based on the experiments. But it’s one of those things that we are predisposed to learn, like language. If the child has anything like normal experiences, it is guaranteed to be learned on schedule. However, if the child were deprived somehow of the experiences needed to learn, the fear might never develop. It would be difficult to test this, though, for obvious reasons.
Another little tidbit: Newborns have something called the Moro reflex. If their heads happed to fall back a bit or they otherwise feel like they are being dropped, they fling out their arms and make grasping motions. This reflex goes away as they get older and learn other responses to the fear of falling.
It’s seems something like a throwback to the days when infant humanoids could cling to their mothers.
Don’t most tiny tots crawl right off a bed if you let them?
I say it’s learned.
Saw an experiment with kittens that just got their site (opened their eyes) - they would crawl right over the clear glass section with no fear of falling. A week later they would not venture near it.
Well, iirc, Mohawk and/or Iroquois people were used in the construction of skyscrapers because of a lack of a fear of heights, which I would assume would be basically the same as a fear of falling (and, for what it’s worth, I did a quick snopes search and didn’t find any contrary info, so I’ll assume the tale is true)
This would rather seem to imply that at least some of the fear of falling is learned.
As far as the younger infants and kittens not showing fear
of heights, one must allow the possibility that their
visual sense is not fully developed yet. Perhaps they
are not afraid of the height because they haven’t developed
depth perception yet, but when they do they are
instinctively afraid of the drop they percieve. Or perhaps
not. Also I was wondering if humans (and other animals)
start out fearing everything (except their mother), and must
learn NOT to fear things.
Another thing that makes me believe it’s learned: hold babby or toddler who desperately wants to be put down, and in most cases he or she will try to bend backwards out of your arms so you’ll release them. If fear of falling was instinctual, they’d be afraid of falling on the floor.
Cecil has dealt with the issue of Native American Indians working on skyscrapers
http://www.straightdope.com/classics/a3_060.html
Having climbed and worked on the rigging of ships I have some ideas about this topic. If you think being high up is scary try being high up on the rigging of a ship rolling like crazy. I think there may be a predisposition but it is learnt and can be unlearnt. If you force yourself to sit high up in the rigging for long periods you will see you gradually feel more comfortable.
Most skydivers will confirm that after making a parachute jump fear of heights goes down. Sometimes it takes 1-2 jumps, sometimes a few dozen, but eventually it goes away. That’s when people start to get hurt. Of course if fear does not go away you will not stick around in the sport. Most people will admit a slight adrenaline surge when they go out the door, even after thousands of jumps.
I have heard (anecdotal evidence) that some people are not afraid when they make that first jump, but they seem to be slightly off in other ways too. There are stories floating around that if you have nightmares of falling and make a parachute jump, the dreams go away. Puns intentional.