Why is yawning "contagious"?

What is a yawn and why do people yawn? Yawning has fascinated people for hundreds of years, and many superstitions and myths have been created over the years.
Today we know that a yawn is a reflex of inhalation and exhalation that draws more oxygen into the bloodstream. A reflex is a built-in physical reaction that people often do not have control over. A yawn is often associated with a person being tired, but this is not always the cause for a yawn. People yawn for many reasons including stress, boredom, emotion and over-work.Have you noticed that yawning seems to be contagious? If one person yawns, this appears to cause another person to yawn. Researchers have found that 40-60% of people who see a picture of someone yawning will yawn themselves. Even reading the word YAWN can make people yawn.
Although this “contagious” yawning behavior is not understood, it has been suggested that this could be the result of an unconscious herding behavior — a subtle way to communicate group behavior, such as when a birds follow the behavior of one bird, and all rise together as a whole flock.
The proximate cause for contagious yawning may lie with mirror neurons, i.e., neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g., language acquisition. Yawning may be an offshoot of the same imitative impulse. A 2007 study found that young children with autism spectrum disorder do not increase their yawning frequency after seeing videos of other people yawning, in contrast to typically developing children. This supports the claim that contagious yawning is based on the capacity for empathy.
Maybe a yawn is a signal to the group that it’s time to go to sleep. Or if someone yawns when they’re bored, it may be a sign to change the topic of conversation.
Yawning is not limited to humans. Animals of all types yawn. If you have a dog or cat, you’ve probably seen your pet yawn several times. Even some birds yawn such as cockatiel parrots, Adelie penguins and Emperor penguins.
Some of the more scientific explanations of why we yawn are caused by physical needs. One likely explanation is that the yawning reflex is triggered when our blood needs more oxygen. The deep breath helps replenish the levels of oxygen in our blood. Another common theory is that the yawn help regulate our body temperature. Other hypotheses suggest that the same chemicals in our brain that affect our moods and emotions cause us to yawn.
Some people think that yawning is rude and suggests that you are bored or uninterested. Superstitions about yawning have been around for hundreds of years and stem from the ancient Greeks. A common superstition says that a person must cover his or her mouth when yawning so that the soul does not escape through the mouth. Other superstitions say that yawning is a sign that danger is near. Today most people do not believe these superstitions and know that a yawn is a physical reflex.
Did you yawn while reading this article?

Sources: http://www.chevroncars.com/learn/wondrous-world/yawn-contagious