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It's theorized that it's the brains defense mechanism.
A yawn is caused by the lack of oxygen reaching the brain (usually caused by shallow breathing when tired). However, this can also happen if you are suffocating as well.
So when you see someone yawn, your brain receives a signal that tells it that someone within your vicinity is not getting enough oxygen, and as protection, it incites you to yawn to be sure that you have enough yourself.
P.S. Sorry if someone already posted that. I didn't have time to read the whole thread.
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