How many friends do you need?

Everyone's suddenly interested in ‘Dunbar's Number’ and why the number of friends we can have is limited to about 150.

Despite Facebook and other social network Internet sites, which allow people to have hundreds of so-called friends, he says that in fact, most people's social network sites contain only 100-150 names, and most exchanges are directed at a small inner core of less than half a dozen.

It's not his first brush with the Press. Dunbar's Number first surfaced in 1994, he thinks, following an article he wrote for New Scientist. Being head of Oxford University's Institute of Social and Cultural Anthropology, he is intrigued by social networking in the media, and how the same story resurfaces every so often, only to be copied by all the other newspapers, TV and radio news.

“It's fascinating to think about how networks work. It's like publishing — it”s clear they have no idea which books will sell, so they try to persuade people to write another that’s just like one that sold well.”

It all started with his research into feral goats in Scotland. He moved on to chimpanzees and other primates, looking at why they groomed each other. Was it for hygiene, or to become friends?

“I found a very nice relationship between brain size and grooming group size. If humans were like primates, the size of our social groups should be limited by the size of our brains.

“What we didn’t appreciate at the time was that primate groups are multi-layered. It turns out that humans are no different from any other primates, and 150 is one of those layers. It seems to have a critical resonance, because it’s the number of people you have personal relationships with; those you know as people.”

The circle of 150 contains an inner core of four to six intimates, and about 15 close friends. There are also 1,500-2,000 you might recognise, because you have seen their photograph, for instance, but never met.

So how did he discover these numbers? “Sometimes you get it by asking people, ‘Who would you go to in times of stress or crisis, to put your life back on track?’ The circle of 15 is well understood; this is why team sports, juries and disciples tend to be groups of about 11 or 15. You can also ask people who they would feel upset about if they died tomorrow, or who do they contact at least once a month. The intimate group of five, you tend to contact about once a week.”

What about Princess Diana, whose death seemed to affect thousands of people very deeply?

“This is where it gets interesting. For instance, online dating and email exchanges can lead you to think you know the person well. The cues are much better when you meet someone in person. This is a scale effect, so it”s better with video conferencing, but the difficulty doesn”t go away.”

He added: “The problem with Internet sites is that you risk falling in love with yourself, because you create an image and you are not given any evidence to counter false impressions.”

He admits that, even without the Internet, we are all capable of seeing lovers with a rosy tint.

“But still real life intrudes,” he said. “In the end, there”s nothing like face-to-face interaction and doing stuff together, having a chat or a drink in the pub, playing bridge or going salsa dancing. That is how we establish relationships. No amount of online interfacing will provide that.”

As well as being sought after by journalists, Prof Dunbar is also in demand by the telecoms industry, because Dunbar’s Number limits the amount of memory needed for address books in mobile phones, computers and online gaming. But he says that, despite Facebook, texting and Twitter, relationships survive only if you reinforce them by occasional face-to-face contacts.

* How Many Friends Does One Person Need? a collection of Prof Dunbar’s popular science writing, is published by Faber at £14.99.