About ten years ago, before the explosion of social networking, anthropologist and psychologist Robin Dunbar conducted research on the Christmas card sending habits of the English. Dunbar wanted to know roughly the size of a person’s average social circle – remember, this was pre-Facebook. He figured that the effort required to buy, sign, address and post envelopes was a rough proxy for the value of a person’s social network
Because of the need to keep these posts brief, I’ll jump to the conclusion by noting that the total population of the households of the average card mailing was 153.5, or roughly 150. Dunbar was not surprised.
For two decades anthropologists and researchers like Dunbar have consistently discovered groupings of 150 members nearly everywhere they looked, according to a recent article in the January issue of Bloomberg Businessweek. Hunter gatherer societies tend to cluster around 150 members. In Western military history, the size of the smallest autonomous military unit, the company, has generally hovered around 150. Hutterites, a sect similar to the Amish, always split when they grow larger than 150. Even the makers of Gore-Tex employ a management structure that breaks a division up and builds a new office whenever it exceeds 150 people.
Dunbar posits a simple explanation: in the same way that we humans can’t, say, breathe underwater or run a 100 yard dash in three seconds, we also cannot maintain many more than 150 meaningful relationships.
The reason — and the size of our relationship span — appears, researchers believe, to be related directly to brain size. According to Dunbar’s writing in the Journal of Human Evolution, while it’s nice to be smart, big brains demand a lot of energy and require years to grow to full size. Dunbar theorizes that “big brains evolved to solve the problem of social life.” Living in large groups confers significant advantages, the chief one being protection against predators. But living together is also difficult, what with competition for food, security and access to mates. This is particularly true of course for primates, like us.
Taking it one step further, Dunbar concluded that there is a dizzying amount of data to be processed in the social network he describes. When Dunbar (in 1992) plotted the size of the neocortex of each type of primate against the size of the group it lived in, he found the larger the neocortex, the larger a social group it could handle. Basically, as brain size increases, so does social group size. When Dunbar plugged in the human neocortex ratio to determine a predicted human group size, he came up with 147.8.
While Dunbar was not the first to posit that “social dynamics explained the evolution of higher intelligence,” his simple math was impressive and convincing to many in the scientific community.
This “Dunbar Number” has made Robin Dunbar an intellectual celebrity. He’s authored books, spoken at TED and is sought out by the likes of the BBC and others while remaining employed at Oxford. There are more nuances to the 150 number (for example, the scale of numbers beginning with a group of “closest” friends numbering 3 to 5, widening to a close circle of 12 to 15 — those who death would be devastating to us — and so on), but those are for another longer post perhaps.
For now, it’s simply worth noting that “like all good theories, it explains constraints, constraints in nature” explains venture capitalist Jerry Murdock, an investor in Path, a service that allows people to post photos from their phones, message one another and comment and search through friends’ material. No surprise then that Path limits its networks to no larger than 150 people.
It might also suggest that if you have many more than 150 Facebook friends, you’re probably pushing the boundaries of your definition of “friends.” Or just really popular.
Oh, and in honor of Valentine’s Day today… tell your closest friend you love them. I know I will.