Sorry to get personal, but how large is your - oh, what's the word - your guild? Or supergroup, clan, allegiance or other moiety of players in your massively multiplayer online game (MMOG) of choice? Does it have, perhaps, around 150 players?

And with how many of those players do you feel close, such that playing with them increases your immersion in the game? No more than a dozen, right?

Okay, maybe you're different. But if you ask around among the other players, most of them will hang out with fewer than a dozen people online, and they hardly ever belong to a group larger than 150 people.

That's not a limit in the game's code or its interface. Some people suggest the limit is hard-wired in your brain.

***

In 1993, Dr. Robin I. M. Dunbar, an evolutionary psychologist then at the Human Evolutionary Biology Research Group of the University College London anthropology department, was studying the behavioral ecology of primates, the relationship of primates to their environment. Dunbar analyzed numerical data from primate studies conducted worldwide. He observed certain "defining behavioural characteristics," such as "the time devoted to social interaction, the level of social skills and the degree of tactical deception practiced."

Dunbar noticed a given species always formed groups no larger than a certain size, and a member of that group always had about the same number of grooming partners. For instance, chimpanzee tribes have a maximum size of about 50 chimps, and each chimp has no more than two or three partners. Dunbar proposed maximum group size depends on the size of the primate brain's neocortex (the part that thinks) - the larger the neocortex, the larger the group. "Animals cannot maintain the cohesion and integrity of groups larger than a size set by the information-processing capacity of their neocortex."

Extrapolating to human societies based on the size of the human neocortex, Dunbar theorized human beings naturally form groups no larger than about 150 (147.8, actually) and "cliques" of about a dozen. Dunbar's paper, "Coevolution of neocortical size, group size and language in humans," appeared in Behavioral and Brain Sciences Vol. 16, issue 4.

The figure of 150 people has become known as "Dunbar's Number." The Number is a conjecture so far, supported only by statistical and anecdotal evidence. Dunbar, now at the University of Liverpool School of Biological Sciences, is conducting a 10-year study that may offer firm proof in 2008.

But some have already seized on the Number as proven fact. Malcolm Gladwell, in his bestselling 2003 business management book, The Tipping Point: How Little Things Can Make a Big Difference, popularized Dunbar's Number (or, as he called it, "the Magic Number One Hundred and Fifty"). Similar books by Duncan Watts and Mark Buchanan advance the Number as a foundational structure for organizations and marketing campaigns.

On the web, humorist David Wong used the Number to launch a funny (but not-safe-for-work) screed about the "monkeysphere" - "the group of people who each of us, using our monkeyish brain, is able to conceptualize as people. ... [I]n our monkey brains the old woman next door is a human being, while the cable company is a big, cold, faceless machine. That the company is, in reality, nothing but a group of people every bit as human as the old lady, or that some kind old ladies actually work there and would lose their jobs if enough cable were stolen, rarely occurs to us."

Today, Dunbar's Number has gained currency among sociologists, anthropologists, managers and - increasingly - online game designers.

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