The optimal size of a group is first determined by the function of the group. A group will need the requisite skills, abilities, experience, and manpower to accomplish its goal. In addition to function, there are cognitive and social underpinnings which influence the optimal size of a group. Studies suggest optimal group size tends to cluster around 5, 15, 50 and 150.

Dunbar’s Number

Robin Dunbar, an anthropologist and psychologist, proposed that 150 is maximum size of a group that can maintain stable social relationships, where each member knows every other member and how they relate to each other. For a group of this size to remain cohesive, as much as 42% of the group’s time would need to be devoted to maintaining relationships which is can only happen when the group is in close proximity and with strong survival pressures. See the Dunbar’s Number wikipedia article which provides additional background information and optimal social networks (the paper) which provides insight from modeling. 

Dunbar also identified a sequence of sizes for cognitively efficient social groups: 5, 15, 50, 150, and 500. Each of these sizes represents different layers of social relationships, varying in the strength of these relationships.

Other Theories

The largest group size that can maintain effective coherence, deep trust and highly efficient communication is generally considered to be relatively small, typically under 10, though the exact number can vary depending on the context and the nature of the group’s interactions.  It has been suggested that online groups might be able to function effectively at higher numbers of participants. There are a variety of theories and research topics in psychology, sociology, and organizational studies that can help identify factors which determine the optimal size of a group:

1. Small Group Theory / Dynamics (3-20 members): Research in small group dynamics typically finds that groups of around 3 to 8 members are most effective for tasks requiring close collaboration and communication. In such small groups, members can form strong interpersonal relationships and maintain a high level of mutual understanding.  When the goal is to foster deep connections and effective communication in a community setting the community can grow to around 20 people with smaller groups within the larger community can ensure everyone feels heard and supported.
2. Communication Network Theory: As group size increases, the complexity of communication channels also increases exponentially, making efficient communication more challenging. Smaller groups allow for more direct and clear communication.
3. Collective Intelligence Theory: This theory would support a group size that balances diversity of thought with the ability to make collective decisions effectively, which could align with a smaller to medium-sized group.
4. Steiner’s Model of Group Effectiveness: This model would favor a group size that maximizes productivity (in terms of community support and cohesion) while minimizing losses due to potential group conflicts or inefficiencies.
5. Social Loafing Theory & Ringelmann Effect: These theories suggest that in smaller groups, members are more likely to contribute actively and take responsibility, which is vital in a community.

Other Observations

When it comes to teams taking on a technical task, Amazon has popularized the idea  of the Two-Pizza Teams, that is effective meetings and teams are small enough that 2 pizzas can feed the gathering.

Over the years Jackie and I have experimented with the optimal size of a dinner group which has the goal of people getting to know each other and share life. We found that 4-6 seemed to be optimal in providing diversity while providing opportunities for each person to contributed. Conversations naturally flow with effective turn taking. When the group grows to 8-15 we have seen that a skilled facilitator are able maintain a good environment, but without a facilitator the quieter people often don’t have an opportunity to contribute, and side conversations will start.

Highly functional start-ups can easily grow to 50-150 people and keep good alignment of collaboration across the company. After this alignment tends to pull apart as subgroups interests come into conflict. As a group grows it’s impossible for everyone to recognize, much less “know” each other. Individuals gravitate to only knowing members of their team.

The most effective strategy when a company is more than 150 people is to identify people who can be interfaces for the other teams. The works best if you have a personal relationship with one or two people in each of the teams who you can build trust with, and who can represent this team to you. This often works best if the person is not the manager or director of that team. Having relationships with people in other teams naturally occurs when people join an organization which was smaller than 50 people, when the company was small enough to know everyone. As the company grows larger, these early team members typically know at least one person in every major group. People who join companies when they are larger, this has to be an intentional activity. Smart companies provide opportunities for people to interact with members of other teams personally, in a context free of work pressures.

Often a mission is too large to be taken on by a small team. A common solution to this is build a hierarchy of teams with middle managers with one or more directors or executives that leads the group. When (not if) projects fall behind expectations there is a temptation to add additional people. This always ends badly. See the classic book The Mythical Man Model for a description of this.

The most extreme example I saw of how a large team can be hampered by size was a skunkworks project of around 8 researchers at Xerox PARC that built a product based on a series of tools that were in common use at PARC. At the same time a “clean sheet” product team was assembled to create a very similar product. The product team grew to 250 people and failed to deliver a useful product after two years of work. Eventually that team grew to 750, then downsizes and refocused. After five years they finally produced a product that was mostly competitive with the product the researchers had been selling to customers for the previous five years.

Often the best solution for taking on larger missions is to have a network of autonomous teams which collaborate. This approach was eloquently discussed in Team of Teams by General Stanley McChrystal.

Further Research?

The following are papers and books which examine issues related to how the size of groups impacts the groups function.

1. What is the Ideal Team Size to Maximize Productivity
2. how to assemble a team with dunbar’s numbers
3. dunbar’s number in social architecture
4. Effective teams for the military is a life or death matter. The core of the military are fire-team and squad. A discussion of how function has impacted the size of a squad.
5. The Magical Number Seven, Plus or Minus Two: Some Limits on Our Capacity for Processing Information by George A. Miller: While not about group size per se, this seminal paper on cognitive processing limits indirectly informs theories about optimal group sizes, especially in contexts requiring intensive communication and coordination.
6. Group size and the trust, cohesion, and commitment of group members Shane Drew Soboroff. Dissertations that examines how group size affects trust, cohesion, and commitment in group members.
7. Why individuals in larger teams perform worse by Jennifer Mueller: examines who as group size increases, individual’s perception of support shrinks.
8. Communication and Group Decision Making Randy Hirokawa & Marshall Poole. This book provides insights into how group size affects communication and decision-making processes within small groups.
9. Research of George Homans. Best know for his paper “The Human Group“. This is a foundational text in understanding group dynamics.
10. Group Dynamics by Donelson R. Forsyth: This comprehensive textbook. Several sections, such as group coherences, collective behavior, and communication talk about how size has an impact.
11. The Five Dysfunctions of a Team by Patrick Lencioni: Addresses team dynamics, indirectly touching upon group size issues.
12. Designing Team-Based Organizations by Susan Albers Mohrman, et al.: Discusses organizational design and team size in the context of knowledge work.
13. Group Creativity: Innovation Through Collaboration by Paul B. Paulus and Bernard A. Nijstad: Explores how group size affects creativity and innovation.
14. A Meta-Analysis of Group Size Effects in Electronic Brainstorming: More Heads are Better than One – Alan R. Dennis & Michael L. Williams
15. The Difference: How the Power of Diversity Creates Better Groups, Firms, Schools, and Societies by Scott E. Page: Addresses how diversity in groups, influenced by size, affects performance.
16. The Wisdom of Crowds by James Surowiecki: This book discusses how large groups can be remarkably intelligent and are often smarter than the smartest people in them