The people whom you are not too embarrassed to disturb are people you can have a meaningful relationship with. ''They know where you fit into their social world and you know where they fit into yours,'' Dunbar says. ''You know them well enough to understand what they might be thinking, rather than simply interpreting their behaviour.'' It's often said that you're a lucky person if you can count the number of good friends you have on one hand. And Dunbar - the author of How Many Friends Does One Person Need? - says the core number of intimate relationships people have is limited to between five and seven. ''They're the ones who you'd go to in times of great crisis or distress,'' he says.

The maximum number of people for whom we can put names to faces - including the weatherman and other celebrities whom you can recognise, but don't know - is about 1500, says Dunbar. But there are differences between men and women, and people can move in and out of the various layers between friends, acquaintances and recognisable strangers. Dunbar's ideas are based on research on the social structure of primates. And while it's obvious that having a group of close associates is a good way to reduce the chances of being attacked and eaten in the wild, mystery still surrounds the Dunbar Number and the other figures that feature in human communities - everything from the size of football teams, to business structures and the way an army organises itself for battle. Not all ''friends'' are considered equal. The Dunbar Number consists of layers. After the intimates, the next layer expands to about 15 people (including the core five). ''These are people that if they suddenly died tomorrow, you'd be devastated,'' he says.

The next layer grows to about 50 people. Anthropologists aren't exactly sure how to define this group, except to say that in traditional societies, such as Australian Aborigines, this would have been the average number of people in an overnight camp site. The reason humans can only have about 150 friends is because the human brain can't cope with the complexity of more, Dunbar says. However, beyond this number there are two larger layers - of about 500 and 1500 - of other people we know but not well enough to consider them friends. These are the types of people that plump up our Facebook accounts, says Dunbar. Why the layers are roughly scaled in multiples of three is unclear, but there is a lot of variation between individuals, he says.

''Everybody is not stuck with these specific numbers. These are simply the averages across the population. Women tend to be better at social things, so they tend to have slightly bigger groupings at each level. There are differences due to personalities, too. ''But we do know the average emotional closeness declines between layers.'' Dunbar's Number and the layers around it stem from the professor's research on primates. Despite the fact that Dunbar spent most of his childhood in East Africa, he had no interest in wildlife or natural history as a boy. It was a chance encounter with some baboons on a university expedition to Ethiopia years later that sparked his interest in primates. When trying to explain why monkeys spend so much time grooming, Dunbar had the idea that it could be related to how big their groups were.

It was here that he found a direct relationship between their average group size and the size of their neocortex - a part of the brain of mammals that is responsible for social skills. Primates (which includes humans) have very large brains compared with their body size. ''This couldn't have been an accident,'' Dunbar says. ''Brains are expensive. They require a lot of energy, especially if they are big.'' So there had to be a reason for big brains to justify their energy expense. For Dunbar, the reason primates developed big brains is to cope with living in groups. But the exact order of big brains and group living is a bit like the chicken and the egg - it is difficult to tell which one came first. ''You can't have a big group without a big brain, and there is no point having a big brain if you don't want to be doing something with it,'' he says.

''Big groups can't be an accidental byproduct of having a big brain - it must have been driven by the need to have big groups to solve an ecological problem.'' Dunbar suggests big groups are the result of animals forming pair bonds - or monogamous relationships - for joint parental care. Once primates got the hang of how to form a pair bond, they could use the same communication skills to form non-reproductive relationships - what we consider friendships. And it turned out that friendships or platonic partnerships were a great way to reduce your chances of being eaten. Although it is well established why primates chose to live in groups, why humans developed the same behaviour is less clear, says Dunbar. Despite this, there is clear evidence that the size of the human neocortex is linked to the average number of friends humans have. And whatever the reason humans started living in groups, we are now stuck in them. ''That's what our brains have developed to handle,'' says Dunbar.

Friendships are not the only evidence of Dunbar's groupings in modern society. In the business world, it's a common rule of thumb that companies with fewer than 150 employees work well when organised on an informal, person-to-person basis. But once they grow larger, a formal hierarchy is needed to maintain efficiency, Dunbar says. ''Bill Gore, the founder of GoreTex, one of the most successful of all medium-sized companies, insisted on creating completely separate factory units each with about 150 workers rather than just making his main factory larger when the growth of his business demanded more production - something that I suspect was the key to the success of his empire,'' writes Dunbar in his book. Louise Young, a marketing professor at the University of Western Sydney, says it is certainly the case that structures within a business have a tipping point. ''You get past the point where everyone in a given organisation or department can be directly connected to everyone else,'' she said. Some firms are increasingly aware that there is a natural limit of relationships that people can effectively manage, says Young. This relationship limit also exists between firms and the number of their clients.

''Since the 1980s some firms have been aware and actively seek to keep the number of relationships they have down to some minimal level. ''The rational for this: you can do a better job in business if you have fewer relationships to spend your finite resources on,'' she said. But the Dunbar Number doesn't stop here. Evidence of human groupings within the smaller layers can also be found.

In sporting teams the number of players generally hovers around 11 to 15 - the second layer of the Dunbar Number. This group is also described as the ''sympathy group'' by many psychologists. ''People in team sports need to be able to work together. If they don't have empathy with each other, the team isn't going to do very well,'' Dunbar says. ''You can always tell when a top level football team has fallen out with each other because they play badly. When they're not getting on personally they lose that edge of anticipation.'' Jonah Oliver, a sports psychologist

in Brisbane who works with AFL, NRL and A-league sporting teams, says psychologists believe there is a limit to how many people humans can interact and maintain direct communication with. In games like soccer and AFL the team can split into smaller groups. ''In AFL the backline [players] have to get the ball to the midfield [players] and the midfield have to get the ball to the frontline. They work as separate pockets on the field,'' Oliver says. ''You see very different team dynamics depending on the size of the team.'' And the more people in a sporting team, the more hierarchical it can become, Oliver says. Juries, the cabinets of most governments, and even the number of Christ's disciples in the Bible are also around the same size as sporting teams, says Dunbar. ''They almost always occur in those numbers because people need to work together,'' he says.

Military structures provide another example of human grouping which fits Dunbar's theories well. In the Australian Army the smallest recognised group is the section, which includes eight to 10 people. The next group is a troop with about 30 personnel, followed by a company with 100 people, and a battalion or regiment with about 600 personnel. Beyond that exists the brigade and the division with 3000 and 10,000 to 15, 000 personnel respectively. According to a Defence spokesman, the hierarchical structure is based on a principle called span of command. ''This is the number of subordinates

a commander can effectively command, that is, lead and manage,'' the spokesman says. But despite the endless examples of natural groupings within society, scientists still don't understand why the human brain can only handle about 150 meaningful relationships - instead of some other arbitrary figure. This is a question Dunbar hopes to answer in the next few years.