The PR BlogMedia & PsychologyBehavioural PsychologySocial Group Sizes (The Social brain Hypothesis)

Social Group Sizes (The Social brain Hypothesis)

The science of in-groups in a wired world.

Cover photo by Jerry Silfwer (Instagram)

Humans tend to organise themselves in stable group sizes.

What used to be dictated by physical proximity is now completely rid of any such restrictions.

The internet allows us to find many different types of tribes and many different types of people to “include” in our groups.

Today, we live in a wired world where you can maintain meaningful relationships with individuals without geographical connections. But how large can such groups be?

Let’s take a closer look:

Dunbar’s Number

The most famous group size is probably Dunbar’s number:

Robin Dunbar - Social Group Sizes - The PR Blog - Doctor Spin
Robin Ian MacDonald Dunbar is a British anthropologist, evolutionary psychologist, and specialist in primate behaviour.

150—Dunbar’s Number

Most of you know Dunbar’s Number. It’s based on the idea that every one of us has limited social bandwidth.

“Dunbar’s number is a suggested cognitive limit to the number of people with whom one can maintain stable social relationships. […] No precise value has been proposed for Dunbar’s number. It has been proposed to lie between 100 and 230, with a commonly used value of 150. Dunbar’s number states the number of people one knows and keeps social contact with, and it does not include the number of people known personally with a ceased social relationship, nor people just generally known with a lack of persistent social relationship, a number which might be much higher and likely depends on long-term memory size.”
Source: Wikipedia

Read also: Group Sizes (From Support Cliques to Tribes)

From Support Cliques to Tribes

But there are other notable group sizes apart from Dunbar’s number. These stable group sizes range from support cliques (3-5 people) to tribes (1,000-2,000 people).

Typical Social Group Sizes

Have you ever heard of the social brain hypothesis? 1Zhou WX, Sornette D, Hill RA, Dunbar RI. Discrete hierarchical organization of social group sizes. Proc Biol Sci. 2005 Feb 22;272(1561):439-44. doi: 10.1098/rspb.2004.2970. PMID: 15734699; PMCID: … Continue reading

“The ‘social brain hypothesis’ for the evolution of large brains in primates has led to evidence for the coevolution of neocortical size and social group sizes, suggesting that there is a cognitive constraint on group size that depends, in some way, on the volume of neural material available for processing and synthesizing information on social relationships.”
Source: Discrete Hierarchical Organization of Social Group Sizes

Scientific evidence suggests that people tend to organise themselves not in an even distribution of group sizes but in discrete hierarchical social groups of more particular sizes:

Alas, there seems to be a discrete statistical order in the complex chaos of human relationships:

  • Support clique (3–5 people)
  • Sympathy group (12–20 people)
  • Band (30–50 people)
  • Clan (150 people)
  • Megaband (500 people)
  • Tribe (1,000–2,000 people)

“Such discrete scale invariance could be related to that identified in signatures of herding behaviour in financial markets and might reflect a hierarchical processing of social nearness by human brains.”
Source: Discrete Hierarchical Organization of Social Group Sizes

Read also: Group Sizes (From Support Cliques to Tribes)

It’s the science of in-groups in a wired world.

Online Identities and Interest Groups

I would say I do know 150 people that I’ve spent time with over the years.

But I also know 150 colleagues that I’ve had. I also know 150 people from the public relations industry, for sure. And I know at least 150 social media naturals, and so on.

How does group formation scale in social media?

I appreciate this model by Viil Lid, PhD candidate in Communication & Information Sciences at the University of Hawaii:

How to scale social media marketing.
How we as individuals shift between roles and communities.

When I’m asked what makes the “social media revolution” so special, I always say that never before in human history have we seen human groups forming at such speeds, almost independent of demographic factors.

It’s the amplification of Dunbar’s number at the interest group level — not due to any sudden increase in our capability to sustain more than 150 relationships.

Group Sizes of Sustainable Relationships

The effects of digital spread are likened to viral infections because boundary spanners and individual nodes have relationships in several different types of interest networks.

For each of these networks, Liid once again shows us a model that I’ve been using in several of the seminars I’ve given:

Group Sizes | Behavioural Psychology | Doctor Spin
We can sustain more extensive networks as ties weaken.

How many “Dunbar, number interest tribes” can a single individual maintain?

If we dig deeper into this question, we must also determine the strength of the individual bindings. Interestingly enough, we see Dunbar’s number in action once again:

  • Inner core (3-5 people)
  • Semi-private layer (<150 people)
  • Superficial layer (>150 people)

The Engagement Pyramid

Building trust is a journey from the periphery to the centre. You start any relationship with an individual or a brand by being a stranger.

PR professionals should explore the digital space, not for clicks, memes, or virals but to build and maintain relationships using online social psychology. Social media doesn’t scale linearly, but tapping into different and pre-existing interest groups does.

When creating a campaign, it’s essential to cater to the inner circles, but don’t forget the different layers of the engagement pyramid.

The Engagement Pyramid

The 1% rule of online engagement was mainly an urban legend on the internet. Still, a peer-reviewed paper from 2014 entitled The 1% Rule in Four Digital Health Social Networks: An Observational Study confirmed the 1% rule of thumb.

Engaged publics typically distribute themselves according to a distribution that has been scientifically proven well before the advent of the internet and social media, and supporting sociologists have made observations for centuries.

The engagement pyramid divides publics into three distinct groups:

  • Creators
  • Contributors
  • Lurkers

When studying internet forums specifically, it’s not uncommon to find that 90% of users have never posted (lurkers), 9% are adding only to existing topics and threads (contributors), and 1% are actively starting new subjects and threads (creators).

The engagement pyramid is sometimes called the 1% rule or the 1-9-90 rule.

Read also: The Engagement Pyramid

1 Zhou WX, Sornette D, Hill RA, Dunbar RI. Discrete hierarchical organization of social group sizes. Proc Biol Sci. 2005 Feb 22;272(1561):439-44. doi: 10.1098/rspb.2004.2970. PMID: 15734699; PMCID: PMC1634986.
Jerry Silfwer
Jerry Silfwer
Jerry Silfwer, alias Doctor Spin, is an awarded senior adviser specialising in public relations and digital strategy. Currently CEO at KIX Index and Spin Factory. Before that, he worked at Kaufmann, Whispr Group, Springtime PR, and Spotlight PR. Based in Stockholm, Sweden.


  1. Ah. I tried to comment before when I read this via @zite:disqus except it failed.

    I was meaning to write a similar post. You saved me from doing all the thinking!

    How many groups am I in with 150+ people. 

    Wow. That’s quite a list

    I love jumping spaces from Fashion to CRM to B2B / BI software to board games to consumer web / social media apps.

    I’ve always felt Dunbar was flawed and that Social Media was raising the (dun)bar!

    Thanks for the Tweet. That made it easy to find my way back! You can’t beat a digital footprint.

    Looking forward to reading more of you posts

    • Looking forward seeing some deep digging into Dunbar.

      The Disqus commenting system drives me crazy by the way, I’m looking into switching to LiveFyre, but I have some issues with commenting importing… hopefully I’ll get some support and can fix this soon.

      Big up for sharing by the way, your thoughts really made some ripples.

    • Livefyre’s support is second to none. Just @ mention them with your problem. I use them for me and for listly.

  2. There are some great things going on here, especially getting firm on the notion of scaling. What I most take from here are the two lower numbers (support clique and sympathy group). From my experiences these two zones aren’t paid attention to enough, and they spin out of my essential notions of trianglization as a social media form (always speaking to the invisible 3rd). These are the gestation groups and require specific nourishing I believe. Great article. Lots to be sifted through.

    • Thanks. I’m thinking of creating a strategy template for how this could look on a pice of paper (for a dummy brand). I think that could serve as a good starting point for a discussion on how to implement this kind of thinking wisely.

    • I loved Kevin’s Invisible 3rd notion from the moment he mentioned it. I use the term Lurkers a lot – We’d been into a conversation on G+ about dunbar, which was why I flipped Kevin and Ric the link.

      Thanks for the nudge @mediasres:disqus – Did you ever write a post on the invisible 3rd?

    • Nick, I’ve never written explicitly about the invisible 3rd, as per Social Marketing or Media, but I do believe this forms what I would call “a social molecule”, a fundamental way of expressing oneself such that molecular bonds can be made between yourself, your possible interlocutor, and the 3rd. I would love to write about this though. I’ve written about it only in Philosophical contexts as it relates to Epistemology and also in consulting contexts for instance in training people how to community manage.

  3. Thxs for a great post! I will refer to it in the future. I don’t know what kind of experience you have from small sociatis and how they use social medias mostly Facebook. Where I live (Åland islands) we have already used networking like that you mentioned long time before social media. Now with Facebook it has been amplified and I can see signs that it has started to change our communication behavior.

    I have written about it here, unfortunate is it in Swedish.

    • I know there are quite a few Swedes lurking around here, so your tip on more reading is much appreciated. Also a very interesting parallel to small local communities with a pre-digital existence. Must look into Åland’s history of digital communication.

  4. Very interesting, Jerry! I’d love to read more on this topic, especially an elaboration on what you think might happen from a business perspective when attracting the wrong interest groups into the funnel. From where I look that is most likely the case when a lot of companies are entering social media for the wrong reasons (everyone else is doing it, we need to in order for anyone to know we exist etc). What’s your general advice for businesses defining right and wrong audiences for them, where is a good place to start, so to say?

    • Great input as always, Mattias. I think segmentation is a key element. I see so many discrepancies between the community a brand WANT and the community a brand actually HAS.

      A good place to start is to skip the traditional demographic segmentation. I think marketing gimmicks like DINK (double income no kids) are becoming more and more useless, to be frank. Instead we need to segment on basis a) how people communicate and b) what people communicate.

      Psychographics, incentive mapping, level of engagement, influence, sentiment etc. There’s a lot of data points to work from. If there’s a big enough data set, even weaker tools (like sentiment) provides good indications for creating strategies.

      But I’m just scratching the surface now of course, I would love to collaborate on a post on how to do a segmentation of “publics” using existing tools and data sets.

    • That would be great fun – let’s have a cup together and see where we get!

    • Thanks Viil and as I said on Twitter — thanks for the inspiration and the models! Would love to dive into your research when published.

  5. Thank you for the post and great description of today’s Dunbar’s number. It’s a great reading. I have a question though, regarding the ‘sharing is carrying’ piece you mention in your article. How much should we share with the different layers? Should you say the message should be consistent throughout all 3 of them (inner, semi-private and superficial)? Or perhaps some differentiation should be in order? Cheers!

    • I’ve been thinking about that a lot. On the hand, nothing really beats one-to-one specific communication, but on the other hand resources aren’t exactly infinite. So where and how to find the balance?

      The value proposition to each Interest Group should be consistent, i.e. the messaging regarding what the brand is all about from a consumer perspective. But what you can ask from the consumer/community in terms of engagement should be diversified. For instance, the closest circle you can ask to co-create. The next circle you can incentivise to share and discuss what’s been co-created. And the outer circle you simply give them an online experience. There’s a cool old-school model (see for instance The Engagement Pyramid) which could be useful here.

    • Hey Jerry! Thanks for the reply. Great input. I was actually thinking of something very similar, but was not sure if this would be the way to go, due to the lack of resources. I believe differentiation is very important, but I can’t seem to see it put to work in an online community setting, like Facebook for example.

      I think it is more important to focus on one core message/engagement technique that can relate to everyone in the community and to outsiders of course, rather than creating different levels of engagement. I mean, at the end of the day all members will have access to the same message and they don’t like to be differentiated.

      Of course, you can argue that some specific involvement/co-creation could be achieved through 1 on 1 communication, but that’s hard to achieve in my opinion, not to mention all the resources that will be consumed doing so. Maybe there is a why of having the best out of the two worlds?!


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