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The passage around this formula

The prediction itself has a precise origin and it is much narrower than its reputation. Robin Dunbar’s 1992 paper in the Journal of Human Evolution ran a comparative regression across primate genera and found that group size tracked one variable with real predictive power — the ratio of neocortex volume to the volume of the rest of the brain — while the ecological variables he tested alongside it, tied to diet and ranging behavior, did not [ 1 ] . Stated as a model, the claim was this: $\log(N_{\text{group}}) = \alpha + \beta \cdot \log\left(\frac{V_{\text{neocortex}}}{V_{\text{rest of brain}}}\right)$. Group size as a function of relative neocortex volume, nothing else in the model earning its keep. Dunbar’s interpretation was mechanistic rather than merely statistical: neocortical…

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