── ── Mental model
Dunbar's Number
Dunbar's number (~150) is the cognitive limit on stable social groups the human neocortex can sustain, with nested layers at ~5, ~15, ~50, then outer bands at ~500 and ~1,500. Derived by Robin Dunbar (1992) from primate neocortex ratios. Key implication: below ~150, informal trust coordinates behavior; above ~150, formal hierarchy becomes architecturally mandatory — not a cultural choice.
How it works
Step 1 — Identify layer: headcount → Dunbar layer (5/15/50/150/500/1,500) → distance to next breakpoint.
Step 2 — Audit coordination: how does the group coordinate? (informal trust / formal process / hierarchy) — where are breakdowns?
Step 3 — Diagnose mismatch: does coordination mechanism match group size? (below 50: direct knowledge ok; 50–150: structured trust ok; above 150: formal hierarchy required)
When to use it
- team headcount is approaching or crossing 150 (or 50, or 500)
- someone says "coordination is getting harder," "we lost the startup feel," or "we don't know who does what anymore"
- deciding whether to split a team, open a new office, or add a management layer
When not to use it
group is well below 50 and problems are clearly interpersonal; core issue is incentive misalignment rather than group size (use principal-agent instead).
Worked example
Dunbar 1992 + W.L. Gore & Associates
Robin Dunbar (b. 1947) is a British anthropologist and evolutionary psychologist at the University of Oxford. His 1992 paper originated in a specific observation: across primates, species with larger neocortex ratios live in larger stable social groups. The neocortex, the "thinking" part of the brain responsible for social cognition, appears to be the binding constraint on social group size — because maintaining stable relationships requires tracking each person's identity, reliability, relationship history, and current status relative to every other group member. The cognitive load scales…
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npx skills add deciqAI/knowledge-skills