Group Thresholds

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Essential overview by Christopher Allen in the Life with Alacrity blog, part of a series of three discussing the ideal numbers of participants in groups.




Christopher Allen:

"If a community is too small you'll often have insufficient critical mass to sustain it. Conversely, if it's too large you can end up with a community that's too noisy, too cliquey, or otherwise problematic. These optimal and sub-optimal community sizes appear in strata, like discrete layers of rock. For a community to advance from one strata to the next often takes immense energy.

We can analyze these community sizes in three ways. In this first article I'm going to talk about numerical group thresholds that have been observed in various sizes of tightly-knit communities, while in its sequel I'm going to talk about personal thresholds and how they relate to group thresholds. In my final post, I'm going to consider how power laws and inequalities of participation further complicate these simple values in the creation of larger communities. Together these three articles constitute what I call "Community by the Numbers," a theory of community size.

Though I'm going to point to some studies which support these numbers, in general my goal here isn't to try and prove this theory of community size numbers, but rather to lay the theory out completely.

Groups can clearly exist at any size, from a partnership of two, on upward. However what I'm going to write about here are the threshold values: the ideal numbers where a community seems to function best, and the less than ideal numbers at which a community begins to grow unstable, remaining so until a new threshold number is reached.

I'm also specifically talking about groups that are both tightly-knit and participatory communities. Clearly Ford Motor Company, with 250,000 employees, doesn't match any of these group thresholds. But any self-contained community within Ford probably will (and in fact, it will probably be either a "Working Group" or a "Non-Exclusive Dunbar Group", both terms I'll explain below). Similarly, a non-corporate community that doesn't require everyone to participate won't work quite the same as a community that does require participation from each member (though that's again the topic of the third article in this series)." (


Christopher Allen:

Group of 7

7, "The Working Group". This community size probably runs from about 4-9 members, but 7 is a pretty good average, and one that shows up in multiple studies. This number may well relate to the general rule of seven (original paper), which suggests that 7 is a number that the brain can easily and intuitively comprehend.

It has become increasingly clear that a tightly-knit group of 7 is the first group size which is truly an optimal community size. Groups below this size can function effectively, but risk not having enough manpower to deliver a result that everyone is happy with, or having insufficient viewpoints to avoid group think.

Seven is not only an optimal size for a wide variety of corporate and government committees, it is also a healthy size for a small business and even a good size for a party of close friends. More importantly, 7 is a very comfortable group size as it "feels" relatively natural. At this size members find it easy to get to know the other members of the group, and they're able to function well together in a very intuitive and organic fashion.

Squad and Fire Team An interesting example of this group size is the modern infantry "squad", which consists of two fire teams of 4 people, and a squad leader, for a total of 9 people. Each fire team is is large enough to function on its own, but together the group of 9 can still have effective small group dynamics.

It is typically at this size that the first signs of leadership in a group informally emerge, but the leadership usually isn't overbearing at this level, nor does there tend to be any rebellion against it — perhaps because the group may be too small to elicit multiple leaders.' (

Group of 13

Last Supper 13 — "The Judas Number".

A group size of 13 doesn't represent a threshold ideal value, but rather a threshold nadir. It is one of the points where groups can change behavior and risk becoming dysfunctional. There's one of these nadirs beyond every group threshold, where the previously harmonious group dynamics become more difficult. I've chosen to highlight this specific number because it's a point that small communities often hit, particularly as entrepreneurial organizations try to grow above their startup beginnings.

(I should note that 13 isn't a precise number, but rather one offered because it's in the right range and because it's poetically easy to remember. The exact number occurs somewhere between 9 and 25, but I suspect it is worst in the range of 12-15.)

In a group of this community size no one ever feels like they get a fair share of time. Studies show that at this size participants underestimate the amount of time they contributed to the conversation, and thus will come out feeling like they were unfairly ignored despite having a fair share of the conversation. Groups of this size risk people being lumped into categories and ceasing to be trusted as individuals. In addition, problems start with the development of "too many chiefs," yet there is not enough enough variety of non-chiefs for them to direct. Furthermore, multiple leaders may struggle for hierarchical status, increasing the conflict in an already troublesome group.

If your community is unfortunately stuck at this nadir, one of two things usually occurs.

Most commonly, the group shrinks. This could be because participants unhappy with the group dynamics abandon it; or it could occur in a more organized way with the unwieldy large group breaking into two or more smaller groups. For example, a terrible group of 13 could become two more functional groups of 6 and 7.

Alternatively, more energy could be expended. This could be in the form of more formal organization, rewards for participation, or more time to be casual and socialize in order to shake off the tensions of this size group. Though these efforts don't usually change the size of the group, they can improve its dynamics.

Energy could also be spent to help push the group up to the next threshold. Though this could occur naturally — for example if the group focuses on a topic of particular interest that causes new people to continually be added. In addition, in order to grow a group to a new threshold it often requires the efforts of more than one leader to succeed.

A group size of 13 isn't necessarily bad, just more difficult. Anthropological studies show that primitive hunting tribes often temporarily broke into "bands" of this size — my presumption is that the value of having that many people hunting together outweighed the social costs of the group. It is interesting that most juries are made up of groups this size. I believe that the social dynamics of this size of group with all new members creates some tension among the jurors, which may serve justice to make sure that all sides are considered by the jury without falling into groupthink. However, from my experience, the interpersonal conflict in a jury can also slow down the deliberation process and cause much frustration among the participants." (

Dunbar 50

Non Exclusive Networks 50 — "The Non-Exclusive Dunbar Number".

"More properly this group size falls in the range of 25-75 participants, but it seems to feel the most natural in the range of 50-60. Studies of the sizes guilds in online games support this hypothesis. For instance, based on graphs of the guild sizes in Ultima Online, groups have a median of 61 members. Similar numbers hold true in studies of a more recent game, World of Warcraft.

I call this value the "Non-Exclusive Dunbar Number" because it matches the lower end of a threshold that Robin Dunbar set for group sizes. However, at this size it applies to mostly non-exclusive groupings, which includes the above mentioned online guilds, many employee communities, and the majority of social gatherings that manage to rise above the size of a Working Group. Groups of this size can be serious or take up a lot of time, but in general they are not exclusive — they don't tend to be the only group that individual participants are involved in.' (

Dunbar 90

90 — "The Dunbar Valley".

"As Non-Exclusive Dunbar Number communities grow, they reach a point where increased time obligations and the noise of socialization required to keep the group cohesive requires a much more serious commitment from the participants. Like the Judas Number, the Dunbar Valley is a threshold nadir where more energy is required to keep a tightly-knit community together; either the community agrees to a higher level of commitment and grows to the next level, or the community splits apart.

I've found this to be true when growing a small business — where it is too small for any middle-management, but the sub-groups are too large for one person to manage effectively. I've also seen this with more ephemeral groups, such as when a small conference that worked well at 60 participants tries to grow and finds at at 100 participants they can't sustain a high enough intimacy level.

Another illustration of the Dunbar Valley is the history of the ancient Roman "century", a grouping that was originally 100 soldiers. However, as the years went by, centuries tended to decrease in numbers to only include 70 or 80 soldiers. This might well be due to Non-Exclusive Dunbar constraints: even in a very devoted group of military men, there was still the need for relationships with other century groups, with support staff, and with camp followers, ultimately lowering the attention that could be spent on the century itself." (

Dunbar 150

150 — "The Exclusive Dunbar Number".

"Robin Dunbar got much of the discussion of group thresholds started with his article, "Co-Evolution Of Neocortex Size, Group Size And Language In Humans." However, as I've written previously, and as I've described in this article, Dunbar's group threshold of 150 applies more to groups that are highly incentivized and relatively exclusive and whose goal is survival.

Dunbar makes this obvious by the statement that such a grouping "would require as much as 42% of the total time budget to be devoted to social grooming."

The result of the grooming requirement is that communities bounded by the Exclusive Dunbar Number are relatively few. You will find hunter/gatherer and other subsistence societies where this is a natural tribe size. You'll also find these groups sizes in terrorist and mafia organizations.

As we step up toward higher group thresholds, more and more time is required to simply keep the group going. You see this in depictions of mafia life — in the TV series The Sopranos a lot of time is spent dining, hanging out, and drinking together. That is part of that 42% social grooming time required for that intense of a survival group.

It is possible for a large company to force groups up to this size by expending lots of energy (which is to say money) to keep it healthy. Apple did this during the invention of the Macintosh, the first OS X operating system, and the iPhone, but the intensity required of such large teams is not sustainable for long periods of time.

Without that extra energy, few modern tightly-knit communities can reach this threshold, or else can't hold it for very long. Instead they fracture into groups of individual interest (even if they continue to "meet" in the same real-world or online forum), which are more than more likely to be bounded by the Non-Exclusive Dunbar number.

Given the difficulty in even arriving at the Exclusive Dunbar number, it may well be the highest limit of all for a tightly-knit community. Beyond this limit, communities are less cohesive, less trusted, and less participatory (and the topic of my third article in this series.)" (

More Information

  1. Dunbar Number