I always figured it was a similar reason to why we say “Ouch” when we get hurt, as a social signal to others saying, first, that they might need to pay attention and, secondly, that you might need help.
[QUOTE=Tom Waits]
When you hear sweet syncopation
And the music softly moans
T ain’t no sin to take off your skin
And dance around in your bones
[/QUOTE]
…
Of course not. It actually happened during the ceremony. (I couldn’t pose something like that if I tried!)
As for whether it’s a laugh or shock, to me, it’s an “oh my god!” type of covering of the face, not so much supresssing laughter, but, either way.
The hand-over-the-mouth gesture goes over better than a fist pump.
I wonder if we can learn something from the natural reaction of children?
Her’s a video clip of a 10 year old girl , when she is shocked to see her father(a soldier) come home from overseas unexpectedly.
First she opens her eyes wide, then her mouth, then she grabs at her chest, then covers her mouth and face, and then there are tears of joy.
The grabbing movement is also instinctive…so maybe it and the mouth covering are related: when you are shocked, you naturally focus all your energy inwards on your own emotions, and on yourself. So you use your hands to cover yourself, to make a sort of barrier, separating your private emotions from the rest of the people around you.
Nice vid! I still think its at least partially learned behaviour. In this case it looks as if she does open her mouth as a purely biological reaction and after a second or two the thought ‘I’m not supposed to have my mouth open’ starts sinking in, so she covers it up.
It’s not just scream.
Watch every single marriage proposal on YouTube (I dare you :)).
What do women do when shown the ring?
What does my wife do to my annoyance every time I begin a sentence like “I got a call from the doctor…”? (It was about a bill from an old check-up.) What does my wife do every time since the last umpteen years, since I know something like that is coming and continually think I will forestall that, begin “Listen, this is no big deal, but…”
She’s wound a little tight.
A fun little excersize is to go into actor mode when trying to analize an emotional response. I tried to recreate the situation and see what kind of emotions are evoked.
I think it has something to do with knowing your mouth needs to say something but guarding against saying the wrong thing. " I don’t know what to say!"
If I was told something big just got screwed up I would clasp my forehead because I know I have to think but am drawing a blank. I am absorbing the situation.
I will rub the back of my head when I actually start the decision making process because there is tension comming from my neck.
I have no idea if any of these are accurate but this is what I got from reenacting.
Try yawning in public without covering your mouth. I think you’d feel vulnerable and uncomfortable. I propose this is innate, not a learned reaction.
You need to have better ideas. 
You would be wrong.
In circumstances where I don’t consciously care what people think I’ll yawn my head off uncovered.
The mere fact that “consciously care” is the criterion means it’s taught behavior; if it were innate, it’d happen without conscious intent.
A 10 year-old girl has been through enough societal conditioning that what she does is no longer a “natural reaction.”
Ladies used to have a fainting couich as a piece of furniture in their house. And, they were taught how to use it. Or they would excuse themselves from the dinner table and retire to their rooms, announcing “I have a slight headache”. Probably because they were going to throw up, in what is thought to be the precursor of what is now called Bulemia, the only form of protest that was available to a proper young lady.
The popular media has reinforced the behavior. A proper up-speaking valley girl will announce, in a loud, clear, well-enunciated voice “Oh Emm Gee” when suddenly taken by surprise. Or say, quietly and without intonation or facial expression, “ew” when something is totally beyond disgusting.