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Dunbar's Number and the Reason for Our Big Brains

Dr. Thomas Swan has a PhD in psychology from Otago University.

All of the great apes are social creatures with large brains.

All of the great apes are social creatures with large brains.

Humans have especially large brains, which is no surprise when considering the manner in which we have mastered our environment. In fact, humans have brains that are about 1/40 the size of our bodies. This is one of the largest ratios in the animal kingdom, although some small birds, rodents, and ants have larger brain-body ratios due to the small size of their bodies.

The first clue to understanding why humans have big brains can be found in our evolutionary family tree. Humans are closely related to the other great apes, which is supported by the fossil record and our striking genetic resemblance. This great ape or "hominid" family includes the chimpanzee, bonobo, gorilla, and orangutan.

A part of the brain that is especially large in hominids is the neocortex, which forms the outer layers of the brain and, thus, is a more recent evolutionary addition. The image below compares various primates in terms of their neocortex size relative to another part of the brain that is more ancient (closer to the brain stem) called the medulla. Apes have the largest ratio, with humans (homo) the largest of all.

Neocortex to medulla ratio in prosimians (e.g., lemurs), platyrrhines (e.g., monkeys), OWM (e.g., baboons), and hominoids (e.g., gorillas), including humans (homo).

Neocortex to medulla ratio in prosimians (e.g., lemurs), platyrrhines (e.g., monkeys), OWM (e.g., baboons), and hominoids (e.g., gorillas), including humans (homo).

The Importance of Social Group Size

The second clue to understanding why humans have big brains is that all of the great apes (including humans) interact socially within their species: they live together in large groups of individuals, communicate with each other at various levels of complexity, and cooperate to achieve collective goals. This cross-species sociality suggests that our common evolutionary ancestor exhibited and provided the basis for this behavior.

Anthropologists such as Robin Dunbar have shown that the other great apes (i.e., not humans) live in social groups of no more than 80 individuals. In instances where a group pushes against this limit, it disperses or fractures through infighting, which occurs because apes are not able to maintain relationships with more than 80 individuals in the time they can allocate to being sociable. Given that apes must eat, sleep, and forage, they can only allocate about 20% of their time to social activities.

As well as their demanding environment, apes have another disadvantage: they cannot communicate verbally (beyond basic calls) and their most common way of socializing is to groom each other. While this physical contact releases endorphins that calm the ape, engendering feelings of warmth and good-will that can strengthen group solidarity, it is an inefficient mode of communication that limits their ability to socialize with large numbers of individuals in the already limited time they have for this activity.

Language and speech give humans a more efficient way to build social bonds.

Language and speech give humans a more efficient way to build social bonds.

Although humans also engage in physical behaviors to facilitate good will, group solidarity, and intimacy (e.g., shaking hands), we are able to communicate verbally using language, which is more efficient. Language allows us to talk to many people at the same time and to convey a greater quantity and quality of information to the people around us (and while our hands are free to perform other tasks).

In other words, we can make better use of our social time than apes, which allows us to sustain social groups of up to 150 individuals. This is the (approximate) maximum number of meaningful, personal relationships that it is possible for a human to maintain. Although this number sounds terribly specific, it is supported by a wealth of anthropological evidence (see video below). The research that gave us this number was performed by Robin Dunbar in 1998, and it is therefore known as Dunbar's number.

Anthropological Evidence for Dunbar's Number

The Social Brain Hypothesis

Once humans had language, we suddenly had a wealth of new ways to socialize. We could advertise our qualities to potential mates, seek advice from peers, deceive our competitors, and police others who would try to deceive us. The complexity of our social relationships grew to reflect the differences we now see between ourselves and the other great apes.

However, this complexity required mechanisms within the brain to process the information that we were receiving through verbal communication. Our brains were simply not equipped to deal with this quantity of information, which needed to be understood and used to promote adaptive behavior. The part of the brain that took up the challenge is called the neocortex, which, as you read about earlier, is particularly large in the great apes.

As in the other great apes, the neocortex deals with social information (visual and auditory), emotional processing, memory, learning, and planning. The human neocortex built on this foundation, becoming larger and more powerful. New synapses were forged to accommodate an increasing quantity and complexity of social relationships. The result was a neocortex that makes up 76% of the human brain; the highest percentage in the animal kingdom.

Thus, humans have big brains because of our large neocortex, which evolved to process social information, and, while other animals such as whales and elephants have bigger brains due to their much larger bodies, these animals do not have a large neocortex. Even among primates, the complexities of human social relationships mean that our neocortex is the largest (see below), and the relationship between neocortex size and social group size has also been observed in bats, carnivores, insectivores, and toothed whales.

The bigger a species' neocortex, the bigger its social group.

The bigger a species' neocortex, the bigger its social group.

Why Evolve to Process Social Information?

Now that we know how our big brains evolved, the question of why this happened becomes much easier to answer. Like most things that come about through evolution, our ability to form large social groups became part of human life because it was adaptive: that is, it helped us to survive and reproduce in a competitive environment.

Essentially, people who were able to utilize verbal communication to build larger and more cooperative social groups were better at surviving and reproducing than people that didn't. They had greater security against rivals and predators and a greater ability to hunt or fight cooperatively. Thus, any mutations for a large neocortex were naturally selected because those without the mutations died more quickly and were less able to pass on their genes. This process eventually led to everyone having a large neocortex.

In sum, the essential finding of Robin Dunbar's social brain hypothesis is that language allowed humans to drastically increase the efficiency and methods of their socializing, leading to social group sizes as large as Dunbar's number (150). This required a larger neocortex to compute the increased volume of socially relevant information coming from speech. Compared to the other great apes, with whom we share a common ancestor, this increased our brain size drastically in relation to our body size. In other words, humans have big brains because we are uniquely social creatures.


  • Dunbar, R. I. M. (1998). The social brain hypothesis. Evolutionary Anthropology, 6, 178-190 (PDF).
  • Dunbar, R. I. M. (2004). Gossip in evolutionary perspective. Review of General Psychology, 8(2) (PDF).

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2012 Thomas Swan


Thomas Swan (author) from New Zealand on September 15, 2015:

Hi Jurjen, I've searched through all the papers I've downloaded on the subject and can't find it either. I first wrote this 3 years ago and may have gotten the image from my lecturer's powerpoint presentation. That would have been Claire White. There are similar but different images available on the web that might be just as good. Sorry I couldn't be more help. -Tom

Jurjen van der Helden on September 15, 2015:

Dear Thomas Swan,

I've tried to find the correlational figure in Dunbar's paper you referred to, but I can't find it. I would like to use it in a book we're writing and need the permission to use it.

Kind regards,

Jurjen van der Helden

Insane Mundane from Earth on March 29, 2015:

The same thing has been said about our genitalia; duh! LOL!