邓巴数
一个人能够稳定维持的社会关系数量存在认知上限，常被估计在 150 左右。
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Dunbar's Number
There is a cognitive limit of about 150 stable relationships one person can maintain.
Takeaways
- Dunbar's number (~150) is the size of a community in which everyone knows each other's identities and roles. Beyond this, our brains struggle to track relationships.
- In software organizations, teams larger than ~100-150 will require more hierarchy or splitting into subgroups to function effectively. Coordination overhead rises sharply.
- Smaller limits exist for closer relationships. Effective working groups might be much smaller than 150, but 150 is an upper bound for a community feeling.
- Strong collaboration happens in groups far below the 150 threshold. Small teams win.
Overview
Dunbar's Number means that an engineering department of 150 might function informally, but as it grows past that, you start needing more formal rules, communication channels, and management layers. Informal knowledge ("I know who to talk to about X") begins to fail, and teams feel impersonal.
The rule encourages designing sub-teams or "teams of teams" that are small enough to work together effectively. Dunbar also proposed smaller social layers: ~5 intimate relationships, ~15 trusted collaborators, ~50 close working relationships, and ~150 stable social connections.
For software teams, high-trust work happens in tiny groups, teams larger than ~10-15 lose cohesion, and the 150 limit applies to departments or communities, not day-to-day collaboration.
Examples
Many startups report that around the 150-employee mark, the culture shifts. You no longer know everyone's name, informal communication falters, and you start needing all-hands meetings and internal newsletters. That's Dunbar's number in action.
The Linux kernel community has thousands of contributors, but within it are maintainers and subsystem teams that are relatively small. Communication is structured into mailing lists by subsystem, so any given person communicates only with a small number of peers.
Amazon's "two-pizza teams" (5-10 people) show that even within 150, effective working teams are smaller units.
Origins
Robin Dunbar introduced this number in a 1992 paper and popularized it in his book *Grooming, Gossip, and the Evolution of Language*. He found a correlation between primate neocortex size and social group size, predicting human size at ~150.
Examples such as historical village sizes, military company units, and communal groups often hover around 100-200, lending credence to the figure.
核心含义
团队规模增加后，成员之间需要维护的关系数量会快速增长。信息同步、信任建立、冲突处理和共同背景都会变得更困难，因此大型组织需要通过小团队、清晰接口和稳定节奏来降低协调成本。
150 不是软件团队的硬阈值，更重要的是认识到人类注意力和关系维护能力有限。
实践例子
一个项目团队从十几人扩展到数十人后，口头同步和临时沟通开始失效。将团队拆成若干有明确职责的小组，并用文档和接口连接，通常比继续增加会议更有效。
来源与边界
人类学家 Robin Dunbar 根据认知能力与社交网络研究提出这一估计。不同任务、文化和工具会改变实际边界，不能把数字当成组织设计的唯一依据。