"It was easy to recognise 'them' when they belonged to an entirely different culture, but how is it done when 'they' belong to our own culture? The language, the customs, the appearance of the internal 'them' is not strange or unfamiliar, so the crude labelling and lumping is more difficult. But it can still be done. One sub-group may not look strange or unfamiliar to another sub-group, but it does look different, and that is often enough.
The different classes, the different occupations, the different age groups, they all have their own characteristic ways of talking, dressing and behaving. Each sub-group develops its own accents or its own slang. The style of clothing also differs strikingly, and when hostilities break out between sub-groups, or are about to break out (a valuable clue), dressing habits become more aggressively and flamboyantly distinctive. In some ways they begin to resemble uniforms. In the event of a full-scale civil war, of course, they actually become uniforms, but even in lesser disputes the appearance of pseudo-military devices, such as arm-bands, badges and even crests and emblems, become a typical feature. In aggressive secret societies they proliferate.
These and other similar devices quickly serve to strengthen the sub-group identity and at the same time make it easier for other groups inside the super-tribe to recognise and lump together the individuals concerned as 'them'. But these are all temporary devices. The badges can be taken off when the trouble is over. The badge-wearers can quickly blend back into the main population. Even the most violent animosities can subside and be forgotten. An entirely different situation exists, however, when a sub-group possess distinctive 'physical' characteristics. If it happens to exhibit, say, dark skin or yellow skin, fuzzy hair or slant eyes, then these are badges that can not be taken off, no matter how peaceful their owners. If they are in a minority in a super-tribe they are automatically looked upon as a sub-group behaving as an active 'them'. Even if they are a passive 'them' it seems to make no difference. Countless hair straightening sessions and countless eye-skin-fold operations fail to get the message across, the message that says, 'We are not deliberately, aggressively setting ourselves apart'. There are too many conspicuous physical clues left.
Rationally, the rest of the super-tribe knows perfectly well that these physical 'badges' have not been put there on purpose, but the response is not a rational one. It is a deep-seated in-group reaction, and when pent-up aggression seeks a target, the physical badge-wearers are there, literally ready-made to take the scapegoat role.
A vicious circle soon develops. If the physical badge-wearers are treated, through no fault of their own, as a hostile sub-group, they will all too soon begin to behave like one. Sociologists have called this a 'self-fulfilling prophecy'. Let me illustrate what happens, using an imaginary example. These are the stages:

1. Look at that green-haired man hitting a child.
2. That green-haired man is vicious.
3. All green-haired men are vicious.
4. Green-haired men will attack anyone.
5. There's another green-haired man - hit him before he hits you. (The green-haired man, who has done nothing to provoke aggression, hits back to defend himself.)
6. There you are - that proves it: green-haired men 'are' vicious.
7. Hit all green-haired men.

The progression of violence sounds ridiculous when expressed in such an elementary manner. It is, of course, ridiculous, but nevertheless it represents a very real way of thinking. Even a dimwit can spot the fallacies in the seven deadly stages of mounting group prejudice that I have listed, but this does not stop them becoming a reality.
After the green-haired men have been hit for no reason for long enough, they do, rather naturally, become vicious. The original false prophecy has fulfilled itself and become a true prophecy.
That is the simple story of how the out-group becomes a hated entity. There are two morals to this tale: do not have green hair; but if you do, make sure you are known personally to people who do not have green hair, so that they will realise that you are not actually vicious. The point is that if the original man seen hitting a child had had no special features potentially setting him apart, he would have been judged as an individual, and there would have been no damaging generalisation. Once the harm has been done, however, the only possible hope of preventing a further spread of in-group hostility must be founded on personal interchange and knowledge of the other green-haired individuals as 'individuals'. If this does not happen, then the inter-group hostility will harden and the green-haired individuals - even those who are excessively non-violent - will feel the need to club together, even live together, and defend one another. Once this has occurred, then real violence is just around the corner. Less and less contact will take place between members of the two groups and they will soon be acting as if they belonged to two different tribes. The green-haired people will soon start to proclaim that they are proud of their green hair, when in reality it never had the slightest significance for them before it became singled out as a special signal.
The quality of the green-hair signal that made it so potent was its visibility. It was merely an accidental label. No out-group has ever been formed, for example, of people who belong to blood group O, despite the fact that, like skin colour or hair pattern, it is a distinct and genetically controlled factor. The reason is simple enough - you cannot tell who 'is' group O, simply by looking at them. So, if a known group O man hits a child, it is difficult to extend antagonism towards him to other group O people.
This sounds so obvious, and yet it is the whole basis of the irrational in-group/out-group hatreds we usually refer to as 'racial intolerance'...


-- Desmond Morris, The Human Zoo