Dunbar's Number: You Cannot Have Infinite Friends

Dunbar’s number is a cognitive limit to the amount of people with whom you can maintain a social relationship. The number was first proposed by Robin Dunbar in the 1990s, who found correlations between primate brain size and average group social size. Using his findings, he proposed humans can comfortably maintain 150 stable relationships. Primates must maintain personal contact with other members of their social groups to function. The number of these members is limited to the size of their neocortex. Dunbar’s primatological findings aligned with how anthropologists reconstructed ancient human tribes and villages.

Nowadays, Dunbar’s number is of interest to us in statistics, business management, and evolutionary psychology. Developers of social software need to know it, since awareness of how big of a social network any given user will have needs to be taken into account for their software to function. In military, operational psychologists seek similar data to decide on policies regarding the cohesion and morale of units. Malcolm Gladwell discussed this in his legendary book, The Tipping Point. He noted that the leadership in a particular company discovered that if more than 150 employees worked in one building, stuff broke down. The firm began building offices that could only inhabit 150 people with 150 parking spaces.

The number has also been used to study various virtual communities. Dunbar himself studied Facebook in 2010. The Swedish Tax Agency planned to reorganize its functions in 2007 to a maximum of 150 employees per office. In a 2018 Financial Times piece, Dunbar mentioned another nuance. About 5 of these people will be our most important ones to whom we devote 40 percent of our social time. 10 more “Special ones” will receive 20 percent, and the rest of the 150 will get the remaining 40 percent of our time.

Note that Dunbar’s number refers to stable relationships. These are characterized by regular interaction, trust, and mutual interests. Not superficial acquaintances. This is realistic since, in a small village, most people know each other personally. It makes the 150 number feasible. It’s why military units, companies, clubs, and tribes of the past naturally gravitated towards roughly 100 to 200 individuals. Social media has not changed a thing by the way. The quality of all the extra connections we make does not match those maintained via face-to-face interaction. The millions of “followers” you may have might have a tiny fraction of relationships which are genuinely meaningful. They are not part of your 150.

Real-world examples of Dunbar’s number:

• Traditional Communities: indigenous tribes and small rural communities used to and do operate with roughly 100 to 200 members. The social dynamics in these communities align with Dunbar’s number. Everyone is familiar with each other. Trust is ubiquitous. The social network is strong.

• Military Units: historical military formations and modern units often structure themselves into groups that approximate Dunbar’s number (e.g. platoons, companies). These group sizes are effective for communication, coordination, and mutual support. Key for operational success.

• Corporate Structures: many organizations find it effective to structure themselves into many teams (e.g. around 150 people per), rather than one gigantic, monolithic company. Keeping divisions in this range enhances internal communication. There is less bureaucratic crap and delayed decision-making. Imagine how fast a decision would travel to leadership in a 10,000 people department versus a 150-person division. Night and day.

• Educational Settings: classrooms or smaller schools with roughly 150 students exhibit strong bonds. Teachers get to know each student individually. To a certain extent, smaller groups make personalized attention the norm. This, of course, benefits learning outcomes.

How you might use Dunbar’s number as a mental model: (1) optimize department size — if you are tasked with dividing an organization, aim for a size that allows for meaningful interactions (around or below 150 members); (2) quality over quantity — in social networks, clubs, or communities, you should focus on strengthening core relationships instead of increasing numbers, more rapport less to report; (3) digital design — know that online platforms made to mimic Dunbar’s number will perform better, such as features that allow users to create small, interest-based communities or niches within large networks (e.g. Discord servers, LinkedIn or Facebook groups); (4) break it up — consider restructuring larger departments into small and nimble teams, each consisting of around 4-8 members, to optimize each person’s accountability.