n86.
The source for the graph is Robin Dunbar, “Neocortex Size as a Constraint on Group Size in Primates”, Journal of Human Evolution, 20, 1992, pp 469–493. The correlation between the mean group where N is the mean group size and CR is the neocortex ratio (the ratio of neocortical volume to the volume of the rest of the brain). The value of 147.9 indicated by the regression for humans is an extrapolation from the results for the other primates. The use of both major axis and least-squares regression, as well as alternative indices of relative neocortical size, all yield equations that are of about this magnitude. With a neocortical volume of 1006.5 cc and a total brain volume of 1251.8 cc, the neocortex ratio for humans is 4:1. If the same correlation between the neocortex ratio and mean group size holds for humans as for the other primates, the mean group size for humans is 147.9. This correlation, not drawn by Dunbar, has been added in the graph. Log10(N) = 0.093 + 3.389 log10(CR) (r2 = 0.764, t34 = 10.35, p