Friends obey the rule of three

NOAH, it is said, counted the animals into his ark two by two. Perhaps sensibly, he was no doubt thinking in reproductive terms. Had he been thinking socially, he might instead have counted his animals by threes. That, at least, is the message of a recent study suggesting that our social networks have a very distinctive structure based on multiples of three.

We all know that we can distinguish friends from acquaintances by how we feel about them. Friends are those we want to spend time with, whereas acquaintances are those whose company is a momentary convenience. But it seems that we make even finer judgments than this in real life. In one study, a number of people were asked to make a list of all those to whom they sent Christmas cards. On average, 68 cards were sent to households that altogether contained a total of about 150 members.

That number of 150 is interesting, because it keeps turning up in everyday life. It turns out to be about the number of people of whom you would feel comfortable asking a favour. It’s about the size limit that an organisation can be without having to have some kind of management structure. It’s a fairly typical size for villages in traditional societies, and it’s the classic size for the smallest military unit in a modern army that can operate independently (the company).

What’s perhaps more intriguing is that if you look at the pattern of relationships within this group of 150, a number of circles of intimacy can be detected. The innermost group consists of about three to five people. These seem to constitute the small nucleus of really good friends to whom you go to in times of trouble - for advice, comfort, or perhaps even the loan of money or help. Above this is a slightly larger grouping that typically consists of about ten additional people. And above this is a slightly bigger circle of 30 more.

The numbers that make up these circles seem to have no obvious pattern. But if you consider each successive circle inclusive of all the inner circles, a very clear pattern emerges: they seem to form a sequence that goes up by a factor of three (roughly five, 15, 50 and 150). In fact, there are at least two more layers beyond this: there is a grouping at about 500 and another about 1,500.

We are not sure what all of these successive circles correspond to in real life, or why they should increase in size by a multiple of three, but some correspond to very well known groupings. The grouping of 12-15, for example, has long been known to social psychologists as the "sympathy group" - all those whose death tomorrow would leave you distraught.

Curiously, this is also the typical team size in most team sports, the number of Apostles ... and the list goes on. The 500 corresponds to the typical overnight camp size in traditional hunter-gatherers such as the Australian Aboriginals or the San bushmen of southern Africa. And the 1,500 is the average size of tribes among hunter-gatherer peoples (usually defined as all the people that speak the same language, or, in the case of very widespread languages, the same dialect).

It seems that each of these circles of acquaintanceship maps quite neatly on to two aspects of how we relate to our friends. One is the frequency with which we contact them - at least once a week for the inner circle of five, at least once a month for the circle of 15, at least once a year for the 150. But it also seems to coincide with the sense of intimacy we feel: we have the most intense relationships with the inner five, but we have a slightly cooler relationship with the ten additional people that make up the next circle of 15. And successively cooler still are our feelings towards the next two layers (those in the circles of 50 and the 150).

It seems as though there is a limit to the number of people we can hold at a particular level of intimacy. There are just so many boxes you can fill in your innermost circle, and if a new person comes into your life, someone has to drop down into the next lower level to make room for them. Interestingly, kin seem to occur more often than you would expect by chance in each of these successive levels. This isn’t to say that we have to include (or even like!) all our kin, but it does seem that kin get given preference: when all else is equal, blood really is thicker than water and we are more willing to help them out.

Robin Dunbar is Professor of Evolutionary Biology at the University of Liverpool.