THE anthropologist Robin Dunbar visited Trinity College’s Science Gallery last week.
Dunbar is famous for a number which bears his name.
Each one of us knows many people.
However, our important relationships, those which involve "trust and obligation", seldom exceed 150, the Dunbar Number. The idea that there’s a limit to the number of close connections anyone can have, came to him while he was studying grooming behaviour in monkeys and apes.
Massage therapies, manicures and exotic body treatments are big business; people love being groomed. But these services come at a cost. Likewise, an animal’s time might be better spent searching for food or mating than combing another’s fur for parasites. Yet, some primates spend up to six hours a day grooming their neighbours. Dunbar argues that there must be a pay-off for such a huge investment.
Monogamous birds preen their partner’s feathers more often than do those with looser mating arrangements. Could grooming be a bonding mechanism, crucial to the development of strong interpersonal relationships, he wondered? The Machiavellian hypothesis says that animals living in complex social groups use their big brains to process the huge amounts of information they gather about their peers.
The outer region of the brain, known as the neo-cortex, is highly developed in primates and plays a major role in social activity. When Dunbar plotted neo-cortex size, group size and the time spent grooming, he found that they were closely related. Humans are primates, so he extended the graph to make a prediction about us. It suggested that the upper limit for close bonds between humans was 148, which he rounded to 150.
Even if our brains had enough processing power to establish lots of deep liaisons, we would not have enough time to set up them up or maintain them.
But proving this thesis was difficult. Relationships nowadays, with lifestyle stresses mobile phones and Facebook, are very difficult to unravel so Dunbar turned to historical records for evidence.
He found plenty of it. Archaeologists think that hunter-gatherer bands consisted of 30 to 50 individuals, drawn from looser associations of 100 to 200. The supplies of fruit and berries which Stone-age people could collect, and of animals they could kill, would not be sufficient to sustain very large groups.
These tightly-knit communities had a high incentive to stay intact and each member would develop a close relationship with everybody else.
Language, Dunbar argues, may have been invented as a time-saving bonding alternative to the physical grooming indulged in by other primates.
Even with the invention of farming, group sizes did not greatly increase; excavations suggest that the villages of Neolithic farmers had about 150 inhabitants. Food supplies may have been more reliable in settled communities but obtaining enough clean water, and disposing of sewage and wastes, would become problematical as groups got larger.
Nor have village sizes increased much over the centuries. Those listed in the Domesday Book had populations of about 150. Parish registers of the 18th century suggest that villages then were no larger. Modern sporting and social clubs are often of this size. A typical Irish wedding might have 150 guests. That Dáil Eireann has 166 members, and the College of Cardinals has 180, may be coincidences, but are they? Dunbar numbers seem valid for primates. The concept also works for the horse family but breaks down when applied to other mammal groups and to birds. Monogamous birds such as eagles parrots and owls, have larger brains than others but the association is loose. Swans and geese pair for life but their brains are not especially large.
Seabirds are monogamous but the partners go their separate ways for the winter; their attachment is to their nesting site rather than their mate.
Zoologists agree that mutual preening has a bonding role but stress that it has important hygienic and strategic functions also. Birds and mammals need the assistance of others to remove parasites from inaccessible parts of their bodies. Tactile stimulation helps initiate and synchronise mating. Mammals which preen their peers may gain information about the body size and strength of potential opponents, useful if planning a palace revolution or improving their standing in the social peck-order.
a d v e r t i s e m e n t
This appeared in the printed version of the Irish Examiner Monday, May 10, 2010
