Dunbar’s Number

If someone asks “How many friends do you have?” what would be your answer? Some of you might think that it depends on the person being questioned. Some of you might actually try to count and give and approximate value. Some of you might give the number of Facebook friends you have because that summarizes the number of people that you might have met & talked through your life time for a certain extent, which makes somewhat sense. But, in a rare incident one of them will give the answer “around 150!” Now you might be wondering what is so special about 150. That is because it is known as the Dunbar’s number and the whole concept which I’ll be addressing in this article is known as “The rule of 150”.

This number was first proposed in the 1990s by British anthropologist Professor Robin Dunbar, who found a correlation between primate brain size and average social group size. By using the average human brain size and extrapolating from the results of primates, he proposed that humans can comfortably maintain only 150 stable relationships. But it doesn’t tell that it has to be exactly 150. As Dunbar explains, the number can vary due to the gender, social exposure & personality. These 150 people represent the set of people that we constantly co-operate with and anyone out of that might just be another person that we just happen to know. Dunbar explained it informally as “the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar”.

Robin Dunbar

There are two main reasons for this limitation.

1. Cognitive Challenge

2. Time budgeting problem

Cognitive Limitation

This is the biological aspect of the problem. Even though humans own the best & the most complex brain in the animal kingdom it still has its own limitations. According to the research done by Robin Dunbar the number of social relationships a human can successfully maintain is limited. Dunbar 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”.

Neocortex is the layer of the brain responsible for cognition and language. The number of relationships which could be maintained by a normal human is 100–230 as predicted by this theory. In his words “Some years ago I found a connection relating social group size in primates to the size of their neocortex. In fact it was human group sizes given human brain sizes to produce this number 150”. During the speech “How Many Friends Does One Person Need?” given by Robin Dunbar in 2010 at the Royal Society of Arts-London, he stated that this discovery was “entirely unexpected”. The connection between the neocortex size and the number of relationships can be seen among other species such as other primates, & mammals. Social Intelligence Hypothesis states that the need for understanding and managing relationships in the key difference between primates and other animals. As the results of many more experiments which were conducted afterwards using other species and humans have shown interesting results. The following are the conclusions scientists came based on those observations.

· Humans are best at sustaining a large number of relationships because they have the largest neocortex

· The number of good relationships Horses tend to have is the same as the number predicted according to the size of their neocortex

· The results for humans can vary depending on the reasons such as gender, social exposure & personality

· Why humans evolved with time but the other primates did not.

· You need to dedicate a lot of brain power to sustain really close friendships such as good pair bonded relationships

· The lack of social activity may suggest that you don’t like to dedicate that brain power to something else other than what you’re truly interested! In his own words “I don’t need to explain you that sustaining close relationships is hard work, & extremely costly to maintain!”

· Pair bonding species have “unusually big brains!”

Time budgeting problem

Think of the time you spend on the usual activities like sleeping, eating, washing etc. There are 24 hours for a day. Even though you spare time to spend time with a large number of people a week, still there is a limit. We simply don’t have enough time to sacrifice on social interaction. Imagine you interact with 2–3 new people a day for the rest of your life. Average life span of a human is about 79 years. If we calculate the total number of people you interact with in your whole life, it’s just above 86,500. Doesn’t feel small yet? There are about 8 billion (8,000,000,000) in the world. The world population is too large for us to actually get to know each other! It’s just too large!

It can be too much time consuming to maintain and eventually become an issue for us to be more productive. As a college student I know the importance of time. It’s tricky to allocate time for your academics, sports, and other social activities. If you spend more time on social media or hanging out with friends, you might find hard to dedicate time on the important things in your life. It can cause you other troubles. Having a smaller social group is somewhat effective to achieve excellence in whatever you do in life.

Most of the highly successful individuals in the history are known to be people who had fewer friends. It clearly helped them in continuing their valuable work because they had more time than others because they limited their social interactions. We can assume that they became loners because they devoted themselves in to their work rather than spending time with others! This clearly gives an important message. If you need to be more productive you have to work harder than everybody else, & you have to spend your time in an attentive manner in order to be more successful than the vast majority of the society!

“You just don’t have time in everyday life to invest in each of those people to the extent where you can have a real relationship with them… We learned two things, the first on is that we have no idea what relationships are. There’s no grand spectrum of social psychology and someone who spent a lot of time looking at these actually don’t know either! … And the second is quality of relationships is proportional to the time invested on them”

“If you look at the pattern of the relationships in this 150, what you find is that it consists of a series of layers, a bit like ripples of a pond. If you drop a pebble in the pond you can imagine the ripples going out… As you go out the layers of your relationships, you include more people but relationships of lower quality” — Robin Dunbar

The next circle in this interpretation consists of 5300 people. Surprisingly ancient Greek philosopher Plato, (A student of Socrates & the teacher of Aristotle) had predicted this number correct! He said that “The ideal of democracy size is 5300”.


Here’s a list of examples where the number seemed to be accurate throughout the history of mankind.

(Source — Vsauce2)

· A study of the twenty tribal societies with available data showed a mean clan group size of 153

· Surprisingly the average number of Facebook friends is 150–200

· A 2011 twitter study of 1.7 million users found they, maintain a stable relationship with 100–200 individuals

· Exchange of Christmas cards in the UK & the maximum network size was about 153.5

· 2008 survey by The Knot Wedding Network of over 18000 brides revealed an average wedding guest total of 148

· The roman army during the Republic utilized a fighting unit called the maniple with 130–140 solders & officers

· When an NFL team wins the super bowl, 150 rings are awarded

· Modern military companies tops at about 150

· Middle Eastern Neolithic villages dating back to 6000BC usually populated 120–150

· The Doomsday book written in 1086 on surveys by the conqueror king William which is one of the oldest ever reports about population, reveals an estimated English village size around 150

· Even in the 1800s an average English village had about 160 residents

· Today the Hutterites, and Amish communities split groups if they exceed 150

· Bill gore split his Gore-Tex factories by 150 to keep his employees functioning in personal cooperative relationships.

We all have our “Currently active relationship” circle. There may be people trying to get inside that 150 for both good and bad intensions. So choose wisely!



Ponder & Wander... That'll make you an interesting person || Engineering Undergraduate ||

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Isuru Pamuditha

Ponder & Wander... That'll make you an interesting person || Engineering Undergraduate ||