Dunbar's number is a theoretical cognitive limit to the number of people with whom one can maintain stable social relationships. These are relationships in which an individual knows who each person is, and how each person relates to every other person. Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group. No precise value has been proposed for Dunbar's number. It lies between 100 and 230, but a commonly used value is 150.
Dunbar's number was first proposed by British anthropologist Robin Dunbar, who theorized that "this limit is a direct function of relative neocortex size, and that this in turn limits group size ... the limit imposed by neocortical processing capacity is simply on the number of individuals with whom a stable inter-personal relationship can be maintained." On the periphery, the number also includes past colleagues such as high school friends with whom a person would want to reacquaint himself if they met again.
Dunbar’s Number explains why big groups are made of smaller, more manageable groups like companies, platoons and squads – or like branches, divisions, departments and committees.
No human institution can function above 150 members without hierarchies, ranks, roles and divisions. To keep groups together, you fall back on rules and regulations, norms and laws, borders and jurisdictions.
In the wild, it takes a lot of social grooming to get a group of 150 people to cooperate and pursue a common goal. In modern life, you depend on institutional structure.
As Malcolm Gladwell pointed out in “The Tipping Point,” if a company grows beyond 150 people, productivity sharply declines until the company divides its outlying entities into smaller groups.
You function better in a cluster – that way everyone in that cluster is connected to each other and only certain individuals connect your cluster to other clusters.
Dunbar’s number isn’t fixed. It can be increased or decreased depending on the environment and tools you have available.
With better tools – like telephones, Facebook, email and so on, you become slightly more efficient at maintaining relationships, so the number can be larger, but not much larger.
The most recent research suggests even power-users of Facebook with 1,000 or more friends still only communicate regularly with around 150 people, and of that 150 they strongly communicate with a group less than 20.
The Social Web is revolutionizing the way institutions operate, and the way people communicate, but in the end it might not have much of an affect on the core social group you depend on for true friendship.