Why do people yawn which causes others to yawn?

Question

Why do people yawn which causes others to yawn?

Answer 1

(The other guy is right about the lack of Oxygen to the brain... but as for the contagious part...)

Early humans lived in small groups, often in caves or other small shelters.  As you know, in the wild, there are a lot of dangers and there is strength in numbers, hence the groups of humans together.  Also, there was no real language.  When some one started yawning, it was sort of like a cue for the rest of the group to sleep.  The response was a yawn in return.  

Answer 2

It's just a lack of oxygen to the brain.  It's contagious because it is only semiautonomic, which means you can do it if you try.   Its kind of like having to pee when you hear water running.

Answer 3

It's a good question.  I don't know the answer.

It's funny when you're on a bus in the morning or at the end of the day, someone somewhere yawns, and then there's a real life cascade reaction throughout the bus. :-)