The Rescue

Violence is totally alien to people on this planet

In the vast expanse of the South Atlantic, halfway between South Africa and South America, an ancient volcano rises from the sea. This is the main island of Tristan da Cunha, the most remote archipelago in the world, home to around 260 souls. And it’s British.


Tristan da Cunha as seen from space

There is one policeman in the British Overseas Territory of Tristan da Cunha, and he’s pretty bored. His colleagues back in Blighty investigate crimes from burglary to murder, but for Inspector Conrad Glass a stolen bicycle would be the highlight of his year.

The fact is that on Tristan da Cunha, with its one pub and six surnames, there is hardly any crime to speak of. Everyone knows everyone else, and when Inspector Glass is called in it is generally to calm down arguments between neighbours, not to arrest and prosecute criminals.

It sounds like an ideal society, unless you are a policeman. But the relative tranquillity of these islands is not the result of any unusual virtue among the inhabitants, nor has it come about through any political genius. Rather, it is a predictable result of the capacities and limitations of the human brain.

It’s hard to commit crimes against people who you know, and harder to get away with it when everyone knows everyone’s business. Yes, there are people who will do terrible things to their closest friends and family, but these are thankfully rare. A small community isn’t guaranteed to be peaceful, but there is at least a good chance that a small enough group, with effective social sanctions to deal with issues before they become too serious and a token police service as a backstop in case they do, could come as close as possible to a crime-free ideal.

The question is, how small is small enough?

The anthropologist Robin Dunbar noted that, in apes and monkeys, there is a good correlation between the size of the neocortex relative to the rest of the brain and the size of the typical social group. The neocortex is involved in many of the higher brain functions, including sensory perception, language and conscious thought, so it’s reasonable to suppose it plays an important role in managing social relations. The correlation noticed by Dunbar suggests that the processing power of the brain sets a limit on the size of the animal’s social group, much as the processing power of your computer limits the number of browser tabs you can keep open at once.

But Dunbar went a step further. He extrapolated this correlation to humans, who have a larger relative neocortex size than any other hominid. This predicted a human group size of 148, though uncertainties in the data mean you can only really say that the group size is somewhere between 100 and 200.

Regardless of these uncertainties, a slightly rounded figure of 150 has now become known as Dunbar’s Number, and is frequently invoked to explain all kinds of social group size, from the average size of villages in the Domesday Book to the size of Army units – the average standalone unit, the company, averages about 150 soldiers.

Of course, you can be acquainted with far more than 150 people, but this does seem to be number of people with whom you can have a genuine social relationship, defined by Dunbar as people you wouldn’t feel embarrassed about joining for a drink uninvited if you happened to bump into them in a bar. Studies of social networks show that the same considerations apply in virtual space: we can have thousands of Facebook friends, but the number that we have some real connection with, as opposed to vaguely remembering from school and paying no real attention to, is limited to about this number of 150. (Speaking of which, I’ve just checked my Twitter profile and the number of people I’m following is… 151. Sorry, but one of you is going to have to go.)

So groups of one or two hundred people can all know each other, but in itself this isn’t enough to keep the peace. Roughly speaking, there’s about 5% of the population who are willing to commit the most appalling crimes in cold blood against people they know well. Some of these people have some recognised clinical condition such as sociopathy, others are just pricks. Whatever their reasons, we could expect our group of 150 people to have 7 or 8 such individuals. That may seem like quite enough dangerous criminals to have in your community, but bear in mind that whether these potential criminal proclivities translate into actual offending is highly situational. It depends on motivation – do they think they have something to gain from the crime? – opportunity – do events put them in a position where crime looks like a good idea? – and ability – even the most sociopathic three-year-old is unlikely to murder the babysitter, and the infirm elderly tend not to get into bar fights. It’s also worth noting that this figure of 7 or 8 is an average, and will fluctuate from time to time, and from population to population. The rule of thumb for counting statistics is that the standard deviation – a measure of the size of fluctuations – is the square root of the average number, so here the standard deviation would be a bit less than three. So a typical range might be 4 – 11, and it wouldn’t be terribly unlikely to sometimes be zero.

Also, some risk factors for criminality have a hereditary component. In particular, sociopathy – the lack of empathy towards others – has a heritability of around 50%. If, by whatever chance, our initial population has few of the genetic factors involved in sociopathy, then the number of potential criminals in subsequent generations may be much lower than the estimates above. (Though there is a caveat: these numbers for heritability come from recent studies on sample groups from the US: the heritability level in a radically different environment may be much higher or lower.)

So a small community may not be perfect, but it certainly seems like it’s more peaceful than the average Monday morning on the tube, never mind daily life in a high-crime inner-city estate. In which case, why don’t we all live like that?

Well, the historical development of urban civilisation is a bit long for this blog post, but the fundamental point is that larger societies are able to do more, different things, are able to advance and innovate artistically, scientifically, socially and technologically in ways that small groups simply can’t achieve. Our ancestors used stone tools for a million years, and the incremental development was so slow that an untrained eye would be hard pressed to distinguish a Lower Paleolithic axe head from a Neolithic model. By contrast, the distinction between a Roman pilum and a Heckler & Koch G36 assault rifle would be apparent to the dullest Neanderthal.

The science fiction author Charlie Stross recently tried to establish what would be the minimum population required to maintain a technological civilisation of the level we have today. It’s about having enough people for the division of labour and specialisation that makes our society possible. It’s about having people who can build all the things we use, people who can maintain them, extract the raw materials needed for manufacture, distribute them throughout the population. It’s having educators who can train the population in complex tasks, farmers who can provide raw foodstuffs, butchers, bakers and sandwich makers who ensure the population is fed, artists, actors and writers who create the vital cultural life of the society, through simple entertainment or deep reflection. It’s about the bin collectors and sewage workers who stop us all drowning in filth, the clinicians and support staff who keep us healthy and care for the sick. It’s even about the politicians and bureaucrats who keep the whole thing working, and the emergency services who step in when it doesn’t.

Stross comes up with an estimate of about a hundred million people. This is roughly the number of people who would have to settle some alien planet before it could be a self-sufficient technological society like the one its people have just left. It’s a bit of a hand-wavey number, to be honest, but even if it’s too high by a factor of ten or a hundred, it still tells us that if a technological civilisation is to survive, it must be of a substantial size – much too large to be peaceful in the way that tiny populations can achieve.

So what of the Didonians? We are assured they are a peaceful people, and that they numbered only about a hundred – few enough that Bennett could reasonably think he had wiped them out by attacking one small ceremony. But with such small numbers, there is no way they could develop a “new ray” for use in construction. Indeed, even the elaborate spike traps in the cave would probably be beyond them: their design and construction would require significant division of labour and surplus resources.

Mind you, if they’re such a peaceful people… who are the spike traps for?



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