Re: Why do we cover our mouths with our hands when shocked or horrified?

As far as I can discover, although this is as you say an "automatic" response in many people, it is probably a learned one. Just as we learn to cover our mouths when we yawn (traditionally, this was so the evil spirits couldn't enter, but now it has more to do with not wanting to display one's tonsils for all to see), we may also learn to cover our mouths when we see something shocking. In most cases, our parents taught us that it was polite to cover our mouths when we yawn, but we can also learn such behaviors by observation and mimicry instead of by direct teaching (think of little girls putting on lipstick, pursing their mouths just like they've seen their mother or the women in the TV ads do--they probably haven't been taught this behavior).

One line of evidence suggesting this could be learnt is that people cover their mouths for different reasons. For example, people tend to cover their mouths when they are lying, or when they are resentful.

There may also be different behaviors in different cultures. Chinese women cover their mouths as a sign of modesty or when they are uneasy with the present situation,although one could argue that this uneasiness is similar to seeing something appalling.

I suspect that the behavior is learned, and that it may have originated from some medical knowlege about bacteria (if you're looking at something disgusting, it may very well be something dead and crawling with germs) or from the same fear of evil spirits that originally led us to cover our yawns.