Robin Dunbar is an anthropologist at Oxford Unversity and in the 1980s while researching primate grooming habits he noted that there seemed to be a relationship between primate brain size and how large their social groups were. That led to work on humans and the eponymous “Dunbar Number” that concludes that the human brain can only handle around 150 people in their social sphere. Anything beyond 150 is too complex for our brains to handle.

Research over the years has more or less confirmed the Dunbar number based on research of the size of hunter-gatherer societies, military company size (all throughout history) and the effectiveness of business organizations at various sizes.

The Dunbar Number of 150 has further sub-layers. The biggest number, 150, is the number of casual friends we can have – like who you’d invite to a big party.  According to Robin Dunbar: “The 150 layer is the important one: this defines the people you have real reciprocated relationships with, those where you feel obligations and would willingly do favors.” Then there is the closest layer – these are your very best friends. His research finds that you really can only have about 5 close friends -there is only so much time and social capital we can put into relationships and close relationships take time and effort. Then there are two layers in between. It looks like this:

Graphic from SugarCRM

These layers and their general size have been confirmed by looking at data relating to Christmas card lists as well as phone call patterns.

The fabric and clothing company, GORE-TEX, is an example of using the Dunbar Number in business (even though at the time Bill Gore had never heard of the Dunbar Number theory). From NPR.org:

From its modest beginnings, GORE-TEX grew and grew, Dunbar says, until Gore opened up a large factory. That, too, continued to grow. Then one day, Dunbar says, Gore walked into his factory. And he simply didn’t know who everybody was. Gore wondered why this was. “It was his gut instinct,” Dunbar says, “that the bigger a company got, people working for the company were much less likely to work hard and help each other out.” Gore did some counting, and realized that after putting about 150 people in the same building, things at GORE-TEX just did not run smoothly. People couldn’t keep track of each other. Any sense of community was gone. So Gore made the decision to cap his factories at 150 employees. “Whenever they needed to expand the company,” Dunbar says, “he would just build a new factory. Sometimes right on the parking lot next door.” Things ran better this way, Gore realized. In smaller factories, Dunbar says, “everybody knew who was who. Who was the manager, who was the accountant, who made the sandwiches for lunch.” Business was never better. One-hundred fifty, it seemed, was a magic number.

How about social media? Has this affected the number of friends we have? The average Facebook user has 338 Facebook friends (the median is 200), yet a study of Facebook users at Michigan State found that out of their hundreds of “Facebook friends” they on average considered only 75 of their Facebook friends as actual friends. Facebook’s own data shows that while people may have hundreds of friends, most interaction is with far fewer, about 3-10 people on average. While social media allows us to keep up with far more than 150 people, Dunbar says it’s shared experiences and face-to-face interactions that lead to the sort of relationships we call “friendship” and he questions whether given our limited social capital and brain limitations whether social media can expand our number of friends beyond 150.

Robin Dunbar 2012 TEDx Talk: Dunbar TED Talk