A decade ago, the evolutionary psychologist Robin Dunbar began a study of the Christmas-card-sending habits of the English.  Dunbar wanted a proxy for meaningful social connection. He was curious to see how many people a person knew, but also how many people he/she cared about.  After all, sending a card is an investment: most people won’t do it for just anybody.

Dunbar found that the average English household’s network of yuletide cheer was 153.5, or roughly 150.  This was exactly the number he expected.  For decades, he and other researchers had discovered groupings of 150 nearly everywhere they looked. Anthropologists had found the world’s remaining hunter-gatherer societies tended to have 150 members. Throughout Western military history, the size of the smallest autonomous military unit, a company, has hovered around 150.  Studies have shown that even self-governing communes and a remarkable number of faith congregations split when they grew larger than 150. Even a number of firms such as W.L. Gore & Associates (Gore-Tex) have practices where they open new operations when they expand past 150.

It appears that once a group grows larger than 150, people begin to lose their sense of connection.  This appears to be the upper limit for our memories to remember names and faces  … and thus to manage in terms of relationships.  Beyond 150 (some say up to 500) we are able to remember faces, but not much more.

What about other groupings?

Groups of 5 – 7:  This is the size at which social interactions begin to explode but formal structures aren’t required to control.  This makes this size group almost ideal for decision-making … lots of ideas emerge, but consensus is still possible.

Groups of 9 – 25:  This is the size of many work groups.  It also turns out to be the size where most of us maintain special ties and loyalties feeling a “part of the team.”  When the group grows past 15 we begin to feel we aren’t getting airtime and begin to recede from the group.

Groups of 25 – 75:  Groups this size is generally less homogenous and nonexclusive.  Most people don’t feel they “belong” at this size and there is much less group identity.  Organizations often find it necessary to administer and structure themselves for the first time in a formal manner when they reach this size.  We can’t seem to organically organize when we’re this many.

Groups of 75 – 150:  The reality is that most groups this size actually consist of multiple sub-groups.  It takes an increasing amount of energy and time to hold groups this size together simply because we are reaching our outer limits for relationships.  Everything from decision-making to communication requires formal processes and a lot more energy and attention.

Why is this worth knowing?

All of us belong to multiple groups.  A part of what determines how we feel about the group, how we connect within the group, and how we manage the group is controlled by something as basic as its size.  There are even those who claim our online social networks are limited by Dunbar’s number of 150.  How many “Friends” do you have?