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 look at any species of primate — at every variety of monkey and ape — the larger their neocortex is, the larger the average size of the groups they live with.

Dunbar's argument is that brains evolve, they get bigger, in order to handle the complexities of larger social groups. If you belong to a group of five people, Dunbar points out, vou have to keep track of ten separate relationships: your relationships with the four others in your circle and the six other two-way relationships between the others. That's what it means to know everyone in the circle. You have to understand the personal dynamics of the group, juggle different personalities, keep people happy, manage the demands on your own time and attention, and so on. If you belong to a group of twenty people, however, there are now 190 two-way relationships to keep track of: 19 involving yourself and 171 involving the rest of the group. That's a fivefold increase in the size of the group, but a twentyfold increase in the amount of information processing needed to "know" the other members of the group. Even a relatively small increase in the size of a group, in other words, creates a significant additional social and intellectual burden.

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. Dunbar has actually developed an equation, which works for most primates, in which he plugs in what he calls the neocortex ratio of a particular species — the size of the neocortex relative to the size of the brain — and the equation spits out the expected maximum group size of the animal. If you plug in the neocortex ratio for Homo sapiens, you get a group estimate of 147.8 — or roughly 150. "The figure of 150 seems to represent the maximum number of individuals with whom we can have a genuinely social relationship, the kind of relationship that goes with knowing who they are and how they relate to us. Putting it another way, it's the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar."

Dunbar has combed through the anthropological literature and found that the number 150 pops up again and again. For example, he looks at 21 different huntergatherer societies for which we have solid historical evidence, from the Walbiri of Australia to the Tauade of New Guinea to the Ammassalik of Greenland to the Ona of Tierra del Fuego and found that the average number of people in their villages was 148.4. The same pattern holds true for military organization. "Over the years military planners have arrived at a rule of thumb which dictates that functional fighting units cannot be substantially larger than 200 men," Dunbar writes. "This, I suspect, is not simply a matter of how the generals in the rear exercise control and coordination, because companies have remained obdurately stuck at this size despite all the advances in communications technology since the first world war. Rather, it is as though the planners have discovered, by trial and error over the centuries, that it is hard to get more than this number of men sufficiently familiar with each other so that they can work together as a functional unit." It is still possible, of course, to run an army with larger groups. But at a bigger size you have to impose complicated hierarchies and rules and regulations and formal measures to try to command loyalty and cohesion. But below 150, Dunbar argues, it is possible to achieve these same goals informally: "At this size, orders can be implemented and unruly behavior controlled on the basis of personal loyalties and direct man-to-man contacts. With larger groups, this becomes impossible."

Then there is the example of the religious group known as the Hutterites, who for hundreds of years have lived in self-sufficient agricultural colonies in Europe and, since the early twentieth century, in North America. The Hutterites (who came out of the same tradition as the Amish and the Mennonites) have a strict policy that every time a colony approaches 150, they split it in two and start a new one. "Keeping things under 150 just seems to be the best and most efficient way to manage a group of people," Bill Gross, one of the leaders of a Hutterite colony outside Spokane told me. "When things get larger than that, people become strangers to one another." The Hutterites, obviously, didn't get this idea from contemporary evolutionary psychology. They've been following the 150 rule for centuries. But their rationale fits perfectly with Dunbar's theories. At 150, the Hutterites believe, something happens — something indefinable but very real — that somehow changes the nature of community overnight. "In smaller groups people are a lot closer. They're knit together, which is very important if you want to be effective and successful at community life," Gross said. "If you get too large, you don't have enough work in common. You don't have enough things in common, and then you start to become strangers and that close-knit fellowship starts to get lost." Gross spoke from experience. He had been in Hutterite colonies that had come near to that magic number and seen firsthand how things had changed. "What happens when you get that big is that the group starts, just on its own, to form a sort of clan." He made a gesture with his hands, as if to demonstrate division. "You get two or three groups within the larger group. That is something you really try to prevent, and when it happens it is a good time to branch out."