Evidence base
Robin Dunbar, in The Conversation
Dunbar's Number: Why My Theory That Humans Can Only Maintain 150 Friendships Has Withstood 30 Years of Scrutiny
What it found
Dunbar argues that humans can maintain around 150 stable social relationships, a figure derived from fitting the relationship between neocortex ratio and group size across primates and inserting the human value (yielding 147.8). He stresses that the layered structure around that number matters as much as the total: roughly 5 intimates, 15 close friends, 50 friends, and 150 meaningful contacts.
Why it matters for practice
This is the originator's own plain-language restatement of the theory, useful as a citable source rather than a secondary account, but it is a defence of the concept rather than new evidence, so readers should treat it as argument, not fresh data. The layered structure is arguably more actionable for programme design than the headline number itself.