Dunbar numbers and team design

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The Agile Scrum methodology suggests that small scrum teams are more effective in attaining their goals. Amazon’s two-pizza rule also highlights the benefits of smaller teams. The underlying principle is that small teams lead to better collaboration and more agility in execution. There is also a mathematical and social basis for the effectiveness of small teams. In this blog, we will explore the Dunbar numbers and see how these numbers related to the size and effectiveness of software development teams.


Robin Dunbar is a British anthropologist who researched about a primate’s brain size and the size of its social group. Armed with the findings, Dunbar sought to apply the research to human beings. Consequently, in the 1990s, Dunbar suggested that human beings can develop deep trust with 15 people. According to the Wikipedia article:

Dunbar’s number is cognitive limit to the number of individuals with whom any one person can maintain stable relationships

And according to an article in the New Yorker

Judging from the size of an average human brain, the number of people the average person could have in her social group was a hundred and fifty.

The Dunbar number is actually a series of them. The best known, a hundred and fifty, is the number of people we call casual friends.

The Dunbar numbers

Here is a summary of the numbers postulated by Robin Dunbar along with my categorization for each number:

  • Circle of intimacy - this is number of people you have the closest and most intimate connection with. As per Dunbar this number is 5
  • Circle of deep trust - Dunbar’s research shows that you can have deep and trustful relationship with up to 15 people. We will see how this number relates to the size of a team.
  • Circle of active relationships - This is the number of people with whom we can maintain active and meaningful relationships. The Dunbar number is 50. This might map to a department in an organization.
  • Circle of known contacts - This is the number of people with whom you can maintain occasional contact and engage in simple conversations. This might be the right size for a business unit in an organization and the Dunbar number is 150.

According to the New Yorker, these numbers have been tested and validated against groups in armed forces across centuries, modern social networks and even hunter-gatherer groups.

A note about the numbers

Robin Dunbar suggests that human beings have limited cognitive capital. Hence there are limits on the number of meaningful relationships human beings can engage in. However this also means that if one of the relationship requires more capital, it will reduce the overall number of relationships. In other words, depending upon the quality of relationships the numbers may go up or down a bit.

Dunbar’s Number and the span of control for a manager

For a manager there are three levels of connections that require deep trust.

  • His own direct reports
  • His peers
  • His manager

With this background, a span control of 7 seems like an ideal team size for a manager. Assuming all managers in the hierarchy have 7 direct reports, any managers will have 7 (direct reports) + 6 peers + 1 (his own manager). Given the cognitive capital required for modern professional relationship, this total of 14 relationships fits the Dunbar’s number for the circle of deep trust.

While it is quite common to see large teams and hierarchies, according to Dunbar’s research, the cognitive capital will be more thinly spread out for larger groups.

Dunbar’s Number and scrum team size

The latest version of the Scrum guide says:

The fundamental unit of Scrum is a small team of people, a Scrum Team. The Scrum Team consists of one Scrum Master, one Product Owner, and Developers. Within a Scrum Team, there are no sub-teams or hierarchies. It is a cohesive unit of professionals focused on one objective at a time, the Product Goal.

The Scrum Team is small enough to remain nimble and large enough to complete significant work within a Sprint, typically 10 or fewer people. In general, we have found that smaller teams communicate better and are more productive.

While there is no specific number prescribed by Scrum, an article by Jeff Sutherland - the co-creator of Scrum, suggests that the Scrum team size should be kept under 7. This number is close to the circle of intimacy number of 5. In a software development team, the cognitive capital should allow for a slightly larger number of professionally intimate relationships. Even the break up of the 7 member scrum team is suggested to be 5 developers, 1 product owner, and 1 scrum master.

Additional thoughts

Dunbar’s number is based on the limited cognitive ability of human beings. So the natural question is can I have only 5 intimate connections covering both personal and professional lives? If not, then does it mean that I can maintain 5 intimate connections each at personal and professional lives? And further does it mean that cognitively we can separate our personal and professional lives?

I would answer the latter question in affirmative. I believe we can have separate sets of intimate relationships at work and home. The degree of intimacy might vary but it is common to have strong connections with managers, mentors, and coaches. And if anything a good cohort at work is likely to strengthen personal relationships at home.

Similar argument can also be extended for the circle of deep trust. It is quite common for managers to have 15 connections in the circle of deep trust at work. This definitely does not mean that the manager is incapable of having similar connections outside work.