The Tipping Point: How Little Things Can - By Gladwell, Malcolm Page 0,65

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.

As human beings, in other words, we can only handle so much information at once. Once we pass a certain boundary, we become overwhelmed. What I’m describing here is an intellectual capacity—our ability to process raw information. But if you think about it, we clearly have a channel capacity for feelings as well.

Take a minute, for example, to make a list of all the people you know whose death would leave you truly devastated. Chances are you will come up with around 12 names. That, at least, is the average answer that most people give to that question. Those names make up what psychologists call our sympathy group. Why aren’t groups any larger? Partly it’s a question of time. If you look at the names on your sympathy list, they are probably the people whom you devote the most attention to—either on the telephone, in person, or thinking and worrying about. If your list was twice as long, if it had 30 names on it, and, as a result, you spent only half as much time with everyone on it, would you still be as close to everyone? Probably not. To be someone’s best friend requires a minimum investment of time. More than that, though, it takes emotional energy. Caring about someone deeply is exhausting. At a certain point, at somewhere between 10 and 15 people, we begin to overload, just as we begin to overload when we have to distinguish between too many tones. It’s a function of the way humans are constructed. As the evolutionary biologist S. L. Washburn writes:

Most of human evolution took place before the advent of agriculture when men lived in small groups, on a face to face basis. As a result human biology has evolved as an adaptive mechanism to conditions that have largely ceased to exist. Man evolved to feel strongly about few people, short distances, and relatively brief intervals of time; and these are still the dimensions of life that are important to him.

Perhaps the most interesting natural limit, however, is what might be called our social channel capacity. The case for a social capacity has been made, most persuasively, by the British anthropologist Robin Dunbar. Dunbar begins with a simple observation. Primates—monkeys, chimps, baboons, humans—have the biggest brains of all mammals. More important, a specific part of the brain of humans and other primates—the region known as the neocortex, which deals with complex thought and reasoning—is huge by mammal standards. For years, scientists have argued back and forth about why this is the case. One theory is that our brains evolved because our primate ancestors began to engage in more sophisticated food gathering: instead of just eating grasses and leaves they began eating fruit, which takes more thinking power. You travel much farther to find fruit than leaves, so you need to be able to create mental maps. You have to worry about ripeness. You have to peel parts away in order to eat the flesh of a fruit, and so on. The problem with that theory is that if you try to match up brain size with eating patterns among primates, it doesn’t work. There are primate leaf eaters with big brains and fruit eaters with smaller brains, just as there are primates with small cortexes who travel great distances for their food and primates with big brains who stay at home to eat, so the food argument is a dead end. So what does correlate with brain size? The answer, Dunbar argues, is group size. If you