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あくびが伝染する理由は十分には解明されていない。 ここでは代表的な仮説を三つ。
このTED-Ed を見聞きしているだけでも、あくびが出やすくなるから不思議。
　　　　　　　　　　　　　　　　　
易　　05分・・150wpm　　2014/09/07 新出
　
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Oh, excuse me! Have you ever yawned because somebody else yawned? You aren't especially tired, yet suddenly your mouth opens wide and a big yawn comes out. This phenomenon is known as contagious yawning.

And while scientists still don't fully understand why it happens, there are many hypotheses currently being researched. Let's take a look at a few of the most prevalent ones, beginning with two physiological hypotheses before moving to a psychological one.

Our first physiological hypothesis states that contagious yawning is triggered by a specific stimulus, an initial yawn. This is called fixed action pattern（定型行動パタ-ン）Think of fixed action pattern like a reflex. Your yawn makes me yawn. Similar to a domino effect, one person's yawn triggers a yawn in a person nearby that has observed the act. Once this reflex is triggered, it must run its course. Have you ever tried to stop a yawn once it has begun? Basically impossible!

Another physiological hypothesis is known as non-conscious mimicry（無意識模倣）,(or the chameleon effect. This occurs when you imitate someone's behavior without knowing it, a subtle and unintentional copycat maneuver. People tend to mimic each other's postures. If you are seated across from someone that has their legs crossed, you might cross your own legs. This hypothesis suggests that we yawn when we see someone else yawn because we are unconsciously copying his or her behavior.

Scientists believe that this chameleon effect is possible because of a special set of neurons known as mirror neurons. Mirror neurons are a type of brain cell that responds equally when we perform an action as when we see someone else perform the same action. These neurons are important for learning and self-awareness. For example, watching someone do something physical, like knitting or putting on lipstick, can help you do those same actions more accurately.

Neuroimaging(神経画像) studies using fMRI, functional magnetic resonance imaging, shows that when we seem someone yawn or even hear their yawn, a specific area of the brain housing these mirror neurons tends to light up, which, in turn, causes us to respond with the same action: a yawn. Our psychological hypothesis also involves the work of these mirror neurons. We will call it the empathy yawn（共感あくび）.
Empathy is the ability to understand what someone else is feeling and partake in their emotion, a crucial ability for social animals like us.
　　（fMRI ： functional Magnetic Resonance Imaging　機能的磁気共鳴画像法）

Recently, neuroscientists have found that a subset of mirror neurons allows us to empathize with others' feelings at a deeper level. Scientists discovered this empathetic response to yawning while testing the first hypothesis we mentioned, fixed action pattern. This study was set up to show that dogs would enact a yawn reflex at the mere sound of a human yawn. While their study showed this to be true, they found something else interesting. Dogs yawned more frequently at familiar yawns, such as from their owner's, than at unfamiliar yawns from strangers.

Following this research, other studies on humans and primates have also shown that contagious yawning occurs more frequently among friends than strangers. In fact, contagious yawning starts occurring when we are about four or five years old, at the point when children develop the ability to identify others' emotions properly.

Still, while newer scientific studies aim to prove that contagious yawning is based on this capacity for empathy, more research is needed to shed light on what exactly is going on. It's possible that the answer lies in another hypothesis all together. The next time you get caught in a yawn, take a second to think about what just happened. Were you thinking about a yawn? Did someone near you yawn? Was that person a stranger or someone close? And are you yawning right now?