THE human brain can only cope with about 150 friends – despite Facebook and other social network Internet sites offering you the chance to make thousands.

That’s the verdict of Prof Robin Dunbar, head of Oxford’s Institute of Social and Cultural Anthropology.

He says that although some people have hundreds of so-called friends on the Internet, in fact, most people’s social network sites contain only 100-150 names.

‘Dunbar’s Number’ first surfaced in 1994 following an article he wrote for New Scientist.

It started with his research into feral goats in Scotland.

Then he moved on to chimpanzees and other primates, looking at why they groomed each other. Was it for hygiene, or to become friends?

Prof Dunbar said: “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.”

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.

He said: “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.

“In the end, that is how we establish relationships. No amount of online interfacing will provide that.”

Prof Dunbar is in demand from the telecoms industry, because Dunbar’s Number puts a limit on the amount of memory needed for address books in mobile phones, computers and online gaming.

The institute’s latest research involved giving free mobile phone contracts to a group of Sheffield sixth formers, to see how their relationships were affected by moving away to university.

He concluded that mobile phones did prevent relationships from decaying.

But he said that, despite Facebook, texting and Twitter, relationships survived only if you reinforced them by occasional face-to-face contacts.

How Many Friends Does One Person Need? is a collection of Prof Dunbar’s popular science writing, on subjects such as why monogamy is a drain on the brain, and why gossip is good for you, as well as Dunbar’s Number. It is published by Faber at £14.99.