Regularities in the Aggregate Behaviour of Interacting Economic Agents

There are situations in which agents' behaviour or agents' choices depend on the behaviour or the choices of other agents. In these cases we have to look at the system of interactions between individuals and their environment. Schelling (1978) provides examples of such interactive behaviour. People distribute themselves and congregate at parties and receptions, or form crowds at a rally, a riot or a spectacle, without following a single mode of behaviour. Sometimes people want to be close, sometimes spread out; the people on the edge of a crowd may be pushing to get in and the people in the middle are being crushed. The best and commonest examples are from everyday life. People get separated and integrated by sex, race, age, language, social status or by patterns of acquaintance and friendship. Age at marriage and age differences between spouses are affected by the ages at which others marry. Divorce and the prospects of remarriage depend on whether there is a high rate of turnover in particular age brackets. What other people in the same area are doing heavily influences other kind of behaviours such as the choice of the language, the diffusion of rumour, gossip and news, information and misinformation. The same kind of factor explains the formation of mobs and riots, panic behaviour, rules of the road, taste, style and fashion.