Character, then, isn't what we think it is or, rather, what we want it to be. It isn't a stable, easily identifiable set of closely related traits, and it only seems that way because of a glitch in the way our brains are organized. Character is more like a bundle of habits and tendencies and interests, loosely bound together and dependent, at certain times, on circumstance and context.

Have you ever wondered, for example, how religious movements get started? Usually, we think of them as a product of highly charismatic evangelists, people like the Apostle Paul or Billy Graham or Brigham Young, but the spread of any new and contagious ideology also has a lot to do with the skillful use of group power.

Humans socialize in the largest groups of all primates because we are the only animals with brains large enough to handle the complexities of that social arrangement.

The Rule of 150 says that congregants of a rapidly expanding church, or the members of a social club, or anyone in a group activity banking on the epidemic spread of shared ideals needs to be particularly cognizant of the perils of the bigness. Crossing the 150 line is a small change that can make a big difference.

What must underlie successful epidemics, in the end, is a bedrock belief that change is possible, that people can radically transform their behavior or beliefs in the face of the right kind of impetus.

The three rules of the Tipping Point -- The Law of the Few, the Stickiness Factor, the Power of Context -- offer a way of making sense of epidemics. They provide us with direction for how to go about reaching a Tipping Point.

People weren't getting their jobs through their friends. They were getting them through their acquaintances. Why is this? Granovetter argues that it is because when it comes to finding out about new jobs -- or, for that matter, new information, or new ideas -- "weak ties" are always more important than strong ties. Your friends, after all, occupy the same world that you do. They might work with you, or live near you, and go to the same same churches, schools, or parties. How much, then, would they know that you wouldn't know? Your acquaintances, on the other hand, by definition occupy a very different world than you. They are much more likely to know something that you don't. To capture this apparent paradox, Granovetter coined a marvelous phrase: the strength of weak ties. Acquaintances, in short, represent a source of social power, and the more acquaintances you have the more powerful you are.

The Tipping Point is the biography  of an idea, and the idea is very simple. It is that the best way to understand the emergence of fashion trends, the ebb and flow of crime waves, or, for that matter, the transformation of unknown books into bestsellers, or the rise of teenage smoking, or the phenomena of word of mouth, or any number of the other mysterious changes that mark everyday life is to think of them as epidemics. Ideas and products and messages and behaviors spread just like viruses do.

What happens when two people talk? That is really the basic question here, because, that's the basic context in which all persuasion takes place.

If we think about emotion this way -- as outside-in, not inside out -- it is possible to understand how some people can have an enormous amount of influence over others. Some of us, after all, are very good at expressiing emotions and feelings, which means that we are far more emotionally contagious than the rest of us.