Dunbar's Number

Dunbar’s Number suggests that humans can maintain stable social relationships with approximately 150 people. Proposed by British anthropologist Robin Dunbar, this concept has significant implications for organizational design, communication scaling, and team structures within growing engineering organizations.

Key Implications for Team Design

1. Layers of Relationships and Cognitive Limits

Dunbar’s research outlines nested layers of relationship strength, each with distinct limits:

• 5 People: Deep trust (e.g., a CTO's immediate core leadership team or co-founders).
• 15 People: High sympathy (e.g., direct reports or a tight leadership circle).
• 50 People: Close relationships (e.g., a division or group of teams that interact regularly).
• 150 People: The cognitive limit for stable social relationships. Beyond this point, interactions become transactional, requiring formal rules, processes, and bureaucracy to function.

2. Engineering Team Scaling

• Under 50 Engineers: Communication remains relatively fluid. The entire engineering group can align through informal channels and basic rituals.
• At 50–150 Engineers: The transition to dedicated sub-organizations or "tribes" is necessary. Clear reporting lines and communication cadences must be established.
• Beyond 150 Engineers: Sub-groups start to operate as complete silos unless deliberate efforts (e.g., cross-team Guilds, shared platforms, and rigorous documentation) are put in place.

3. The Need for Subdivisions and Pods

• Two-Pizza Teams (Amazon): Restricting individual team size to 8–10 people ensures that the team stays within the high-trust relationship bounds.
• Spotify Model (Squads, Tribes, Guilds): Restructuring teams into autonomous Squads (6–12 people) grouped into Tribes (capped around 100–150 people) is a direct application of Dunbar's limits.
• Aligning with Conway’s Law: Designing small, bounded teams with clear ownership not only aligns with human cognitive limits but also ensures that the system architecture remains decoupled and modular.

Strategic Utility (Why CTOs Should Care)

For technology leaders, Dunbar's Number serves as a guide for when to restructure the organization:

• CTO Span of Control: A CTO should directly manage no more than 5–8 direct reports. Attempting to manage more reduces the quality of one-on-ones and strategic alignment.
• Engineering Manager Span: Engineering managers should ideally oversee 7–10 direct reports. Beyond this range, they lose track of individual developer experience and performance.
• Anticipate Organizational Fractures: When your engineering organization approaches 150 people, expect cultural and operational shifts. Proactively recruit additional leadership layers (VPs of Engineering, Directors) to maintain cohesion.

References

Internal

• Conway's Law - How team organization dictates software boundaries.
• Brooks’s Law - The impact of communication overhead when scaling teams.
• Communication Lines - Calculating communication channel growth.

External

• Dunbar's NumberWikipedia - Detailed explanation and anthropological background.