volume of the neocortex. This suggests that there is a species-specific index of the social group size, computable from the species' mean neocortical volume.

In 1992, Dunbar used the correlation observed for non-human primates to predict a social group size for humans. Using a regression equation on data for 38 primate genera, Dunbar predicted a human "mean group size" of 148 (casually rounded to 150), a result he considered exploratory because of the large error measure (a 95% confidence interval