Smile like a Million Dollars : Rules for a half smile

I smile a lot and think it is very healthy. In fact it might have been Dale Carnegie's book " How to win friends and influence people" that may have inspired me to smile even under pressure.

The topic I want to talk about is a half smile. At first I was offended when people gave me a half smile and abruptly wiped the smile off while you were looking. Now I think there are some secret rules that I am not privy to about the situations where a half smile is warranted. Here is my guess about what the rules for a half smile are :

1. When a stranger is smiling at you. 
2. You think you know this person but you are not sure you will get a smile back
3. If you don't smile the situation can get nasty
4. It's polite to give a half smile 
5. I want to acknowledge the person but do not want to show familiarity

 In my humble opinion if there is a situation where a half smile is warranted then a full smile is better. What do you think are the situations where half smiles are recommended. What are the downsides of a full smile?

As a reward for reading this post here is a slideshow of all the half-smile pics on Flickr ( see at your own risk )