Dunbar's Number, as the limit on the size of both social groups and personal social networks, has achieved something close to iconic status and is one of the most influential concepts to have emerged out of anthropology in the last quarter century. It is widely cited throughout the social sciences,archaeology, psychology and network science,and its reverberations have been felt as far afield as the worlds of business organization and social-networking sites, whose design it has come to underpin.Named after its originator, Robin Dunbar, whose career has spanned biological anthropology, zoology...