How many friends can you have?

According to Robert Dunbar, the director of the Institute of Cognitive and Evolutionary Anthropology at Oxford University: we can only ever have 150 friends at most. And why is that:

"The way in which our social world is constructed is part and parcel of our biological inheritance,” he explains, " Together with apes and monkeys, we're members of the primate family – and within the primates there is a general relationship between the size of the brain and the size of the social group. We fit in a pattern. There are social circles beyond it and layers within – but there is a natural grouping of 150. This is the number of people you can have a relationship with involving trust and obligation – there's some personal history, not just names and faces."

And how did he come to this unique number - now called the Dunbar number? He is a student of evolutionary anthropology, which is the generic study of how we came to be modern humans – how our bodies came to be the shape they are, how our minds came to be the way they are. He says he was working on the arcane question of why primates spend so much time grooming one another, and tested another hypothesis – which says the reason why primates have big brains is because they live in complex social worlds. Because grooming is social, all these things ought to map together, so he started plotting brain size and group size and grooming time against one another. And he got a nice set of relationships and this number of 150. It seemed that human friendship scale would be larger than that of primates. But it turned out that150 was the sweet spot for hunter-gatherer societies all over the world from the Bushmen of Southern Africa to Native American tribes, as well as Amish and Hutterite communities. Perhaps the best example, however, remains the military. All modern armies have a similar organisational structure, mostly developed over the last 300 years by trial and error on the battlefield. The core to this is the company – typically around 120-180 in size – almost exactly Dunbar's Number. As anyone who has been in the army will tell you, company is family, far more so than battalion or regiment.

"The reason 150 is the optimal number for a community comes from our primate ancestors," Dunbar says. "In smaller groups, primates could work together to solve problems and evade predators. Today, 150 seems to be the number at which our brains just max out on memory." The fact is that quite simply, your brain can't keep track of more than 150 individuals in any effective way. It's not a matter of practice or experience. It's biology and neural logistics! 

The theory goes on to suggest that you cannot maintain 150 relationships unless you spend almost half your available time engaging with them. In certain social situations, this can be accomplished and is sometimes necessary (e.g. military), but in common experience, we simply don't have the time to perform the necessary engagement to maintain all of the relationships. In other words, practical experience indicates that anything more than 150 is not achievable. The critical component is the removal of time as a constraint. In the real world, according to research, we devote 40 percent of our limited social time each week to the five most important people we know, who represent just 3 percent of our social world and a trivially small proportion of all the people alive today. Since the time invested in a relationship determines its quality, having more than five best friends is impossible when we interact face to face, one person at a time. Thus on average, we are likely to have five intimate friends, 15 good friends (including the five intimate ones), 50 friends and 150 acquaintances. A relationship's quality seems to depend on how much time we devote to it, and since time is limited, we necessarily have to distribute what time we do have for social engagement unevenly. Put simply, our minds are not designed to allow us to have more than a very limited number of people in our social world. The emotional and psychological investments that a close relationship requires are considerable, and the emotional capital we have available is limited.

While modern society does make it hard to hang on to friends who aren't geographically close, Dunbar says, his research shows family is different.

"Friends, if you don't see them, will gradually cease to be interested in you," he says. "Family relationships seem to be very stable. No matter how far away you go, they love you when you come back."