Primate brain size is proposed to be driven by the demands of tracking relationships in a group rather than by ecological problem-solving.
Dunbar's 1992 analysis found neocortex ratio correlating with group size across primates, and reasoned that keeping track of who owes what to whom is the computationally expensive task. The extrapolation to a human group size of around 150 is the part that travelled; the underlying comparative claim is where the argument is.
In practice
- A manager who could hold the whole team's relationships in mind at twenty cannot at eighty, and starts needing process for what used to be memory.
- An organisation that grows past the point where everyone knows everyone acquires a formal structure whether or not it wanted one.
- Possible explanations
- The explanation is computational load: tracking who owes what to whom in a large group is harder than solving ecological problems, so social complexity drives brain size.
- In its favour
- The correlation between neocortex ratio and group size across primates is real, and the general claim that social life is cognitively demanding has support from experimental work on primate social cognition.
- Against it
- The correlation is disputed on measurement and comparative method, rival ecological accounts fit parts of the data, and a 2021 reanalysis found the derived human estimate far less precise than usually reported.
- Standing
- The correlation is disputed on measurement and on comparative method, and rival ecological accounts fit parts of the data. Dunbar's number is the best-known extrapolation from it.
- Subject
- Sociology & Culture
- Essay
- How societies hold together, sort people, and change