Over the past decade, primatologists' work, studying social behavior in chimps, gorillas, macaques, bonobos and baboons, has led to a unifying theory that explains not only a huge range of behavior. Also, why our brains are so big and what their most essential work is.
The theory, called the Machiavellian-intelligence or social-brain theory, holds that we rise from a lineage in which both individual and group success hinge on balancing the need to work with others with the need to hold our own amid the nested groups and subgroups we are part of.
Robin Dunbar, an evolutionary psychologist and social-brain theorist, and others have documented correlations between brain size and social-group size in many primate species. According to Dunbar, no such strong correlation exists between neocortex size and tasks like hunting, navigating or creating shelter.
Dunbar's eponymous number is 147.8, plus or minus and it is the size of the average human being's social network of friends, as predicted by the size of the average human brain.
Dunbar's "social brain hypothesis" rests on another idea—the theory of mind—which argues that we use our brains to imagine what others are thinking.
Many years ago Mr. Dunbar famously noticed that there is a tight correlation between the size of a primate's brain and the size of the social group its species generally forms. On this basis human beings should live, work and learn in groups of around 150. The neat thing about this prediction was the way it seemed to fit the number of good friends most people have, as measured by the length of address books, the size of hunter-gatherer bands, the population of neolithic villages and the strength of army units. In recent years, Facebook has also seemed to confirm the hunch, with rosters of friends often settling around the Dunbar number.
Above 150, you have to impose complicated hierarchies and rules to command loyalty and cohesion. Below 150, it is possible to achieve these same goals informally based upon personal loyalties and direct person-to-person contacts.
When things get larger than 150, people become strangers to one another. When your group gets bigger than 150, you begin to get two or three sub-groups or clans within the larger group. Above 150 people, there begins to be structural impediments to the ability of the group to agree and act with one voice.
So if you have an organization (like a business, school, church or social club) that is approaching the 150 number, be cognizant of the perils of bigness.
Now Mr. Dunbar, who teaches at Oxford, has taken the argument a step further in work yet to be published, by correlating the size of a specific part of an individual's brain with the size of that individual's social network. He and his colleagues asked volunteers to list the initials of every person they had social contact or communication with over the previous week, before stepping into a magnetic resonance scanner to measure the volume of their "orbitomedial prefrontal cortex." Sure enough, the size of this lobe of the brain correlates well with the size of a person's circle of friends.
Mr. Dunbar's "social brain hypothesis" rests on another idea—the theory of mind—which argues that we use our brains to imagine what others are thinking. So, drilling down further into the physiology of the brain, Mr. Dunbar's team has now found that a rich social network also goes with the ability to reason about others' intentional states. That is to say, people with more friends are better able to understand the intentions and perspectives of other people from one's own.
The more people we need to understand, the bigger the mental engine we will need.
Source: The Wall Street Journal, February 12, 2011