The main facts about group behaviour are briefly the following:
  1. Owing to the limitations of his mind, a human being thinks in categories (or generalizations).  He associates certain abilities and characteristics with physical appearance.  Once certain associations have been established he finds it difficult to change them.  New experiences which tend to contradict them are ignored.
  2. An individual has values which he cherishes, by virtue of which he is a person, which he derives from the various groups to which he is affiliated.  He tends to think favourably of a group of which he is a member (in-group) as this is a source of pride and satisfaction to him.  It follows that the out-group is less worthy of esteem.
  3. The members of a group, by virtue of their common culture, physical traits or interests, tend to think and feel in the same way towards the out-group.  These feelings are likely to be hostile [See (2)].  Attitudes toward an out-group may be evolved within a very short time, e.g., against new immigrants and 'guest workers' from certain countries.  Joint action may be taken to enhance the group's status and wealth against the out-group.  Often very little communication or none at all is necessary to coordinate action.  Schelling [Schelling, T.C.,  The Strategy of Conflict,  Oxford University Press, 1960; p. 54.--Moreh's citation.] discusses in detail the possibility of such coordination (not necessarily in connection with such group action).  One example he gives is the following: a husband and wife separated in a store are likely to meet each other at a place that seems 'obvious' to both.  Marshall [Quoted in Phelps Brown, H., The Inequality of Pay, Oxford University Press, 1977; p. 145.--Moreh's citation.] mentions the case of farmers acting together to prevent raising wages to farm labour as demand for the latter increases, without communicating among themselves.  Each farmer fears the strictures of other farmers if they discover he raised the wage to the new workers.  It also suits him not to raise.  A similar example, though in a different context, is that of boys in mixed schools using high-handed methods to dominate the girls and exploit them.  No communication among the boys is necessary. The minority of boys who protest such 'sexist' behaviour are intimidated.  [Mahony, P., Schools for the Boys?, London: Hutchinson, 1985; pp. 41 - 50.--Moreh's citation.]
  4. Hostility towards the out-group plays an important role in mental life: one's defects are projected on to the members of the out-group, and if the out-group is weaker than the in-group, it is treated as a scapegoat and prejudiced views are formed against it which, curiously, the weaker group tends to believe.  This is an ideology invented by the strong group to justify its maltreatment of the weaker group. 

from J. Moreh, "The Authority of Moral Rules."  Theory and Decision, vol. 27 (1989), p. 263/264.  [Klüwer Academic Publishers]