Dunbar's Number

Overview

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.

Composes with network-effects (different coordination mechanisms below vs. above 150), principal-agent (exceeding 150 multiplies agency problems informal trust can no longer cover), founder-mindset (founder relationship bandwidth peaks at ~150 — first breakpoint demanding shift from doing to delegating structure).

When to Use

• Headcount approaching or crossing 150 (or 50, or 500)
• "Coordination getting harder" complaints with no obvious structural cause
• Deciding to split a team, open a new office, or create a sub-unit
• Designing org structure for a scaling startup; evaluating acquisition integration plans
• Diagnosing why informal culture "felt different" after a headcount milestone
• AI-native startup hiring fast on abundant AI-era capital / AI-adoption demand — "we scaled from 20 to 150 in a year and coordination is breaking"

Not when: group well below 50 and problems are interpersonal; constraint is incentive misalignment (use principal-agent); challenge is growing a network beyond 150 (use network-effects).