---
type: claim
domain: cultural-dynamics
description: "Dunbar's number (~150) is a cognitive constraint on group size derived from the correlation between primate neocortex ratio and social group size, with layered structure at 5/15/50/150/500/1500 reflecting decreasing emotional closeness"
confidence: likely
source: "Dunbar 1992 Journal of Human Evolution; Dunbar 2010 How Many Friends Does One Person Need?"
created: 2026-03-08
---

# human social cognition caps meaningful relationships at approximately 150 because neocortex size constrains the number of individuals whose behavior and relationships can be tracked

Robin Dunbar's social brain hypothesis establishes that primate social group size correlates with neocortex ratio — the proportion of brain devoted to the neocortex. For humans, this predicts a mean group size of approximately 150, a number that recurs across diverse social structures: Neolithic farming villages, Roman military centuries, Hutterite communities that split at ~150, average personal network sizes in modern surveys, and the typical size of functional organizational units.

The mechanism is cognitive, not social. Maintaining a relationship requires tracking not just who someone is, but their relationships to others, their reliability, their emotional state, and shared history. This mentalizing capacity — modeling others' mental states and social connections — scales with neocortex volume. At ~150, the combinatorial explosion of third-party relationships exceeds what human cognitive architecture can track. Beyond this number, relationships become transactional rather than trust-based, requiring formal rules, hierarchies, and institutions to maintain cohesion.

The number is not a hard boundary but the center of a layered structure. Dunbar identifies concentric circles of decreasing closeness: ~5 (intimate support group), ~15 (sympathy group — those whose death would be devastating), ~50 (close friends), ~150 (meaningful relationships), ~500 (acquaintances), ~1,500 (faces you can put names to). Each layer scales by roughly a factor of 3, and emotional closeness decreases with each expansion. The innermost circles require the most cognitive investment per relationship; the outermost require the least.

This has direct implications for community formation and organizational design. Communities that grow beyond ~150 without introducing formal coordination mechanisms lose the trust-based cohesion that held them together. This is why [[trust is the binding constraint on network size and therefore on the complexity of products an economy can produce]] — trust operates naturally within Dunbar-scale groups but requires institutional scaffolding beyond them. It also explains why [[isolated populations lose cultural complexity because collective brains require minimum network size to sustain accumulated knowledge]] — the Tasmanian population of ~4,000 had enough Dunbar-scale groups for some cultural retention but insufficient interconnection between groups for full knowledge maintenance.

For collective intelligence systems, Dunbar's number defines the scale at which informal coordination breaks down and formal mechanisms become necessary. The transition from trust-based to institution-based coordination is not a failure — it is the threshold where design must replace emergence.

**Scope:** This claim is about cognitive constraints on individual social tracking, not about the optimal size for all social groups. Task-oriented teams, online communities, and algorithmically-mediated networks operate under different constraints. Dunbar's number bounds natural human social cognition, not designed coordination.

---

Relevant Notes:
- [[trust is the binding constraint on network size and therefore on the complexity of products an economy can produce]] — trust is the coordination substrate that Dunbar's number constrains at the individual level
- [[isolated populations lose cultural complexity because collective brains require minimum network size to sustain accumulated knowledge]] — network size must exceed Dunbar-scale for cultural accumulation, but interconnection between Dunbar-scale groups is what maintains it
- [[collective brains generate innovation through population size and interconnectedness not individual genius]] — innovation requires networks larger than Dunbar's number, which is why institutional coordination is a prerequisite for complex civilization
- [[Ostrom proved communities self-govern shared resources when eight design principles are met without requiring state control or privatization]] — Ostrom's design principles are the institutional mechanisms that extend coordination beyond Dunbar-scale groups
- [[civilization was built on the false assumption that humans are rational individuals]] — Dunbar's number is another cognitive limitation that the rationality fiction obscures
- [[humans are the minimum viable intelligence for cultural evolution not the pinnacle of cognition]] — the 150-person cap is evidence of minimal cognitive sufficiency, not optimal design

Topics:
- [[memetics and cultural evolution]]
- [[_map]]
