Dunbar’s Number: A Social Education From Monkeys

Dunbar’s number offers some intriguing lessons on our social lives. Monkeys, it turns out, may just provide the education we need to rethink our relationships.

Primates are highly sociable animals. Their relationships inextricably depend on personal contact among their social groups. In fact, such is the importance that non-human primates place on this contact, they have a regular, go-to method for achieving it: cleaning one another.

In the late 1980s and early 1990s, this social grooming became a focus for an anthropologist by the name of Robin Dunbar. Fascinated by this social custom, the Oxford professor had a simple question in mind: why did primates devote so much time to it?

As his efforts to answer this question accumulated, his observations began to drive at a far more intriguing insight. Dunbar started to plot primate brain size and group size together, and a strong correlation emerged.

His conclusion: the number of social contacts a primate can maintain appeared to be limited by brain size. More specifically, the size of the neocortex – the part of the brain associated with cognition and language – appeared to be linked to the size of social groups.

But the research didn’t stop there. Dunbar wanted to extrapolate his findings to human social groups. Could this correlation shed light on the cognitive limits of our social lives? Did we all have a quantifiable social capacity?

As Dunbar gathered anthropological, historical and psychological data, his findings kept pointing with remarkable consistency around one particular social group size. That number has since been widely popularised in statistics, evolutionary psychology and even real-life business management. The number is 150.

What Is Dunbar’s Number?

Dunbar’s number (otherwise known as the ‘rule of 150’) is a suggested cognitive limit to the number of people with whom we can maintain meaningful relationships.

This may sound a bit farfetched, but it’s been taken seriously. Malcolm Gladwell highlights the idea in Tipping Point (UK, US), describing how the company W. L. Gore and Associates started limiting each building’s capacity to 150 employees – it did this after noticing how buildings shared by more than 150 employees brought different social problems. The Swedish tax authority are also said to have restructured their office environment according to Dunbar’s number.

But while 150 is the number that hogs the limelight, the social brain hypothesis actually suggests six numbers, each illustrating a layer of social proximity. This more sophisticated view of our social sphere is worth some attention.

The social brain hypothesis suggests there are six average social group sizes, per the diagram above:

  • Support clique (5 people): Our tightest circle of loved ones, to which we devote around 40% of our social time.
  • Sympathy group (15 people): These are our good friends. On average, we devote an additional 20% of our social time to the additional 10 people in this category.
  • Close network (50 people): A further 35 friends are added to the mix.
  • Personal network (150 people): The number you already know about, this is the supposed average cognitive limit for meaningful relationships.
  • Acquaintances (500 people): These are people we have contact with, but the additional 350 people are not part of our core network.
  • Recognisable (1500 people): When we add 1000 more people to this mix, we have faces we recognise, but people with whom we don’t have a great deal of social contact.

Numbers Aren’t Everything

The layers of social closeness are where the theory gets interesting, but numbers aren’t everything. It’s clear that an introvert might have much less than 50 people in their close network and an extrovert may be sending out party invitations to a network that dwarfs 50 people. It’s also clear that an extrovert doesn’t therefore have a bigger brain than an introvert (sorry to those extrovert readers!).

Dunbar’s core number has also come under a fair bit of criticism. An alternative number of 290 has not gained similar levels of traction in popularity. There are questions, too, about how social media affects these averages. Those with a wide social media reach, for example, may now recognise a number that comfortably exceeds 1500. There’s also criticism of the extrapolation itself, with critics arguing that it doesn’t adequately reflect the impact of nutrition on our cognitive capacities.

What Can We Learn From Dunbar’s Number?

Notwithstanding, the quantification of a social limit still poses some interesting questions. In an age where virtual connections are expanding our social reach but diminishing the quality of our social relations, understanding the limits to the number of meaningful relationships we can maintain has some important implications.

  • We have our cognitive limits: Whether associated with our brain size or not, meaningful relationships require time and cognitive effort. That invariably means you cannot have an unlimited number of meaningful relationships. There is only so much time and cognitive fuel in the tank each day. When we recognise this limiting factor, we take the first step to optimising our relationships to best support what we value.
  • We should conduct an internal HR audit: Knowing we have limited time and cognitive capacity should be impetus to optimise our relationships. As I’ve argued before, we should all take a hard look at our relationships and ask ourselves whether we devote enough time to some and too much to others. This is particularly pertinent when we keep in mind that 60% of our social time, according to this theory, is allocated to 15 people.
  • We should take action: Optimising your social relationships isn’t as brutal as it sounds. Indeed, often both sides have outgrown each other, but we allow proximity and comfort to rule us. Whatever the size of our personal network, be it 150 or 10, it’s never worth holding onto something that doesn’t support what we value.

Cleaning Up

Take what you will from this article, but I’ll close with a simple observation: we can learn a lot from monkeys.

Granted, we’re unlikely to aspire to sit in social groups, picking insects from one another’s hair, but this custom provides some important lessons.

First, personal contact is the ancestral foundation of our social relations. It’s something to aspire to. Face-to-face conversation beats distant virtual connection every day of the week and twice on a Sunday.

Second, there are cognitive limits to the size of social group that primates can maintain. Let us not forget that we are primates too. If nothing else, Dunbar’s number illustrates the importance of recognising our limits.

Third, these limitations should instigate a rethink. Just as monkeys devote their time to cleaning each other, we should devote some time to cleaning up our approach to our relationships.

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