How Many Friends Does One Person Need? Dunbar’s Number and Other Evolutionary Quirks
by Robin Dunbar. Harvard University Press, 2010
If you find relationships challenging to cultivate and maintain, then you are in good company. In his new book, evolutionary biologist Robin Dunbar argues that our ability to manage such complex social connections—love lives, work colleagues, childhood buddies and friendly acquaintances—is what drove humans to develop such large brains in the first place.
Dunbar finds support for this theory, dubbed the social intelligence hypothesis, by observing birds. He recently conducted studies in several species of birds and found a clear link between brain size and relationship type. Birds that mate for life have much larger brains relative to body size, whereas birds that live in promiscuous flocks have much smaller brains. Dunbar speculates that birds with smaller brains have many short-lived partners because they lack the mental prowess to form and maintain more complex emotional bonds.
Dunbar finds that apes and monkeys form lasting bonds and have a particularly big neocortex—a region of the brain that regulates emotions, awareness of others and language abilities. Humans form some of the most intricate and complex relationships of all. And our brains are high maintenance, consuming a whopping 20 percent of our energy.
Judging from human brain size and complexity, Dunbar calculates that a person’s social group should incorporate about 150 people—this is the maximum number of relationships our brain can keep track of at one time. This figure, now graced with the name “Dunbar’s number” takes different types of relationships into account. On one end of the spectrum, we have a core group of about five people we talk to once a week. On the other end, we have a group of around 100 acquaintances to whom we speak about once a year.
Dunbar’s theory may appear to fall short when you think of the masses of people you know at your church or the hundreds of friends you have on social network sites such as Facebook, but Dunbar claims that the more friends you have beyond his number, the less likely you are to know much about them—or if you can even call them friends.