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. In the late eighteenth and early nineteenth centuries, for example, the Methodist movement became epidemic in England and North America, tipping from 20,000 to 90,000 followers in the U.S. in the space of five or six years in the 1780s. But Methodism's founder, John Wesley, was by no means the most charismatic preacher of his era. That honor belonged to George Whitfield, an orator of such power and charisma that, it was said, he once charmed a five-pound contribution out of Benjamin Franklin — who was, of course, the furthest thing from a churchgoer. Nor was Wesley a great theologian, in the tradition of, say, John Calvin or Martin Luther. His genius was organizational. Wesley would travel around England and North America delivering open-air sermons to thousands of people. But he didn't just preach. He also stayed long enough in each town to form the most enthusiastic of his converts into religious societies, which in turn he subdivided into smaller classes of a dozen or so people. Converts were required to attend weekly meetings and to adhere to a strict code of conduct. If they failed to live up to Methodist standards, they were expelled from the group. This was a group, in other words, that stood for something. Over the course of his life, Wesley traveled ceaselessly among these groups, covering as much as four thousand miles a year by horseback, reinforcing the tenets of Methodist belief. He was a classic Connector. He was a super Paul Revere. The difference is, though, that he wasn't one person with ties to many other people. He was one person with ties to many groups, which is a small but critical distinction. Wesley realized that if you wanted to bring about a fundamental change in people's belief and behavior, a change that would persist and serve as an example to others, you needed to create a community around them, where those new beliefs could be practiced and expressed and nurtured.

This, I think, helps to explain why the Ya-Ya Sisterhood tipped as well. The first bestseller list on which Ya-YaSisterhood appeared was the Northern California Independent Bookseller's list. Northern California, as Wells said, was where 700 and 800 people first began showing up at her readings. It was where the Ya-Ya epidemic began. Why? Because, according to Reverand, the San Francisco area is home to one of the country's strongest book-group cultures, and from the beginning Ya-Ya was what publishers refer to as a "book-group book." It was the kind of emotionally sophisticated, character-driven, multi-layered novel that invites reflection and discussion, and book groups were flocking to it. The groups of women who were coming to Wells's readings were members of reading groups, and they were buying extra copies not just for family and friends but for other members of their group. And because Ya-Ya was being talked about and read in groups, the book itself became that much stickier. It's easier to remember and appreciate something, after all, if you discuss it for two hours with your best friends. It becomes a social experience, an object of conversation. Ya-Ya's roots in book-group culture tipped it into a larger word-of-mouth epidemic.

Wells says that at the end of readings, during the question-and-answer session, women in the audience would tell her, "We've been in a book group for two years, and then we read your book and something else happened. It started to drop down to a level of sharing that was more like friendship. They told me that they had started going to the beach together, or having parties at each other's houses." Women began forming Ya-Ya Sisterhood groups of their own in imitation of the group described in the book, and bringing Wells pictures of their group for her to sign. Wesley's Methodism spread like wildfire through England and America because Wesley was shuttling back and forth among hundreds and hundreds of groups, and each group was then taking his message and making it even stickier. The word about Ya-Ya was spreading in the same way, from reading group to reading group, from Ya-Ya group to Ya-Ya group and from one of Wells's readings to another, because for over a year she stopped everything else and toured the country nonstop.

The lesson of Ya-Ya and John Wesley is that small, close-knit groups have the power to magnify the epidemic potential of a message or idea. That conclusion, however, still leaves a number of critical questions unanswered. The word group, for instance, is a term used to describe everything from a basketball team to the Teamsters Union, from two couples on a holiday to the Republican Party. If we are interested in starting an epidemic — in reaching a Tipping Point — what are the most effective kinds of groups? Is there a simple rule of thumb that distinguishes a group with real social authority from a group with little power at all? As it turns out, there is. It's called the Rule of 150, and it is a fascinating example of the strange and unexpected ways in which context affects the course of social epidemics.

There is a concept in cognitive psychology called the channel capacity, which refers to the amount of space in our brain for certain kinds of information. Suppose, for example, that I played you a number of different musical tones, at random, and asked you to identify each one with a number. If I played you a really low tone, you would call it one, and if I played you a medium tone you would call it two, and a high tone you would call three. The purpose of the test is to find out how long you can continue to distinguish among different tones. People with perfect pitch, of course, can play this game forever. You can play those dozens of tones, and they'll be able to distinguish between all of them. But for the majority of us, this game is much harder. Most people can divide tones into only about six different categories before they begin to make mistakes and start lumping different tones in the same category. This is a remarkably consistent finding. If, for example, I played you five very high pitched tones, you'd be able to tell them apart. And if I played you five very low pitched tones, you'd be able to tell them apart. You'd think, then, that if I combined those high and low tones and played them for you all at once, you'd be able to divide them into ten categories. But you won't be able to. Chances are you'll still be stuck at about six categories.

This natural limit shows up again and again in simple tests. If I make you drink twenty glasses of iced tea, each with a different amount of sugar in it, and ask you 10 sort them into categories according to sweetness, you'll only be able to divide them into six or seven different categories before you begin to make mistakes. Or if I flash dots on a screen in front of you very quickly and ask you to count how many you see, you'd get the number right up to about seven dots, and then you'd need to guess. "There seems to be some limitation built into us either by learning or by the design of our nervous systems, a limit that keeps our channel capacities in this general range," the psychologist George Miller concluded in his famous essay "The Magical Number Seven." This is the reason that telephone numbers have seven digits. "Bell wanted a number to be as long as possible so they could have as large a capacity as possible, but not so long that people couldn't remember it," says Jonathan Cohen, a memory researcher at Princeton University. At eight or nine digits, the local telephone number would exceed the human channel capacity: there would be many more wrong numbers.