THINKING ABOUT YAWNS In this excerpt, from the Introduction, I talk about what it means to think of the world in epidemic terms. A world that follows the rules of epidemics is a very different place from the world we think we live in now. Think, for a moment, about the concept of contagiousness. If I say that word to you, you think of colds and the flu or perhaps something very dangerous like H.I.V. or Ebola. We have, in our minds, a very specific, biological, notion of what contagiousness means. But if there can be epidemics of crime or epidemics of fashion, there must be all kinds of things just as contagious as viruses. Have you ever thought about yawning, for instance? Yawning is a surprisingly powerful act. Just by reading the two yawns in the previous two sentences--and the two additional yawns in this sentence--a good number of you will probably yawn within the next few minutes. Even as I'm writing this I've yawned twice. If you're reading this in a public place, and you've just yawned, chances are that a good proportion of everyone who saw you yawn is now yawning too, and a good proportion of the people watching the people who watched you yawn are now yawning as well, and on and on, in a ever-widening, yawning circle. Yawning is incredibly contagious. I made some of you reading this yawn simply by writing the word "yawn". The people who yawned when they saw you yawn, meanwhile, were infected by the sight of you yawning--which is a second kind of contagion. They might even have yawned if they only heard you yawn, because yawning is also aurally contagious: if you play an audio-tape of a yawn to blind people, they'll yawn too. And finally, if you yawned as you read this, did the thought cross your mind--however unconsciously and fleetingly--that you might be tired? I suspect that for some of you it did, which means that yawns can also be emotionally contagious. Simply by writing the word, I can plant a feeling in your mind. Can the flu virus do that? Contagiousness, in other words, is an unexpected property of all kinds of things, and we have to remember that if we are to recognize and diagnose epidemic change. The second of the principles of epidemics--that little changes can somehow have big effects and vice versa--is a also a fairly radical notion. We are, as humans, heavily socialized to make a kind of rough approximation between cause and effect. If we want to communicate a strong emotion, if we want to convince someone that, say, we love them, we realize that we need to speak passionate and forthrightly. If we want to break bad news to someone, we lower our voices and choose our words carefully. We are trained to think that what goes in to any transaction or relationship or system must be directly related, in intensity and dimension, to what comes out.. Consider, for example, the following puzzle. I give you a large piece of paper, 1/100th of a inch thick. (That's a typical thickness). I want you to fold it over once, and then take that folded paper and fold it over again, and then again, and again, until you have refolded the original paper 50 times. How tall do you think the final stack is going to be? If you ask people that question they'll fold the sheets in their mind's eye, and usually answer that the pile would be as thick as a phone book or, if they're really courageous, they'll say that it would be as tall as a refrigerator. But the real answer is that the height of the stack would approximate the distance to the sun. And if you folded it over one more time, the stack would be as high as the distance to the sun and back. This is an example of what in mathematics is called a geometric progression. Epidemics are another example of geometric progression: when a virus spreads through a population, it doubles and doubles again, until it has (figuratively) grown from a single sheet of paper all the way to the sun in fifty steps. As human beings we have a hard time with this kind of progression, because the end result--the effect--seems far out of proportion to the cause. To appreciate the power of epidemics, we have to abandon this expectation about proportionality. We need to prepare ourselves for the possibility that sometimes big changes follow from small events, and that sometimes these changes can happen very quickly.