why are yawns contagious?

Answer 1

They really are not, it's just a head trick.

Answer 2

Yawning is one of the animal kingdom's oldest rites. Researchers have shown that nearly all vertebrates do it—reptiles, fish, birds and mammals alike. Merriam-Webster's Dictionary says that humans yawn because of boredom or fatigue, but there is much more to this ancient behavior.

Scientists maintain that yawning has both social and physiological functions, and may even be useful clinically: Abnormal yawning can be symptomatic of pathology, such as tumors, hemorrhage or drug withdrawal. Researchers know that a system of several neurotransmitters and neuropeptides control yawning, but little is known about the exact mechanism underlying the action.

Until recently, it was thought that only humans and great apes were able to "catch" yawns. While humans yawn in the womb, they don't fall prey to contagious yawning until about two years of age, which suggests a recent evolutionary origin.

In examining why humans yawn, it's helpful to look at testable common knowledge: In addition to yawning when others do, we yawn when we're sleepy. We yawn most often in the morning. We yawn when we're bored. We yawn when we're hungry.
In 1987, Provine corrected the biggest misconception about yawning: that it is brought on by an oxygen shortage or an elevation of CO2 levels. In his study, he subjected participants to air with higher than normal levels of CO2 (3% to 5% versus the usual 0.03%). The experiment showed that neither elevated carbon dioxide nor depressed oxygen levels in the blood caused the frequency of yawning in subjects to change. In a second study, Provine found that having subjects exercise hard enough to double their breathing rate also had no effect on yawning. Together, the studies prove that yawning is not a primary respiratory function.

All of the accepted characteristics of yawning—hunger, boredom, lethargy—have something in common: They are associated with change. In fact, one function of yawning is its importance in changing behavioral state.

Answer 3

The power of yawning is purely unconscious. We still do not know what causes the body to have a reaction like that. The link below has a detailed explaination.