[5] Dunbar explained the principle informally as "the number of people you would not feel embarrassed about joining uninvited for a drink if you happened to bump into them in a bar.
[7] Proponents assert that numbers larger than this generally require more restrictive rules, laws, and enforced norms to maintain a stable, cohesive group.
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 of 100 to 230).
Without language, Dunbar speculates, humans would have to expend nearly half their time on social grooming, which would have made productive, cooperative effort nearly impossible.
[14] Participants of the European career-oriented online social network XING who have about 157 contacts reported the highest level of job offer success, which also supports Dunbar's number of about 150.
The Bernard–Killworth estimate of the maximum likelihood of the size of a person's social network is based on a number of field studies using different methods in various populations.
A replication of Dunbar's analysis on updated complementary datasets using different comparative phylogenetic methods yielded wildly different numbers.
[citation needed][dubious – discuss] Philip Lieberman argues that since band societies of approximately 30–50 people are bounded by nutritional limitations to what group sizes can be fed without at least rudimentary agriculture, big human brains consuming more nutrients than ape brains, group sizes of approximately 150 cannot have been selected for in paleolithic humans.
[24] [dubious – discuss] Brains much smaller than human or even mammalian brains are also known to be able to support social relationships, including social insects with hierarchies where each individual "knows" its place (such as the paper wasp with its societies of approximately 80 individuals[25]) and computer-simulated virtual autonomous agents with simple reaction programming emulating what is referred to in primatology as "ape politics".
Along with the existence of complex deception in small-brained primates in large groups with the opportunity (both abundant food eaters in their natural environments and originally solitary species that adopted social lifestyles under artificial food abundances), this is cited as evidence against the model of social groups selecting for large brains and/or intelligence.