Yawning has been researched by scientists for years, and is still a mystery to this day as to why exactly humans and animals alike do it.
There have been myths over the years that it is a trigger responce from the brain due to lack of oxegyn, but it was later announced that people given direct oxegyn, were just as likely to yawn.
It is normally something that is done early am, or pm, due to fatigue, but can also come at times of boredom.
Yawning is psychological because you can tell yourself not to do it and you won't. This is the reason why it is not contagious The normal feeling human, who can sympathize with anothers feelings, will yawn after another person, out of a person's natural reaction to atmosphere.
This has also proven that schitzophrenics (sp) and people that suffer from severe cases of mania, are less likely to yawn after another person, due to their lack of sympathizing for another person in feelings, and lack of aknowledgement to reality.
This by no means is implying that someone who doesen't yawn after another is a manic...just that it is a psychological act, when in reaction to another's yawn