ABSTRACT
Historians frequently try to understand why groups of people acted together as they did in the past. It is convenient to regard social groups as being of roughly three kinds. First there are aggregates, groups of people who act in a similar way largely on their own initiative, such as voters at an election. Second, there are social groups which are formed when several people commit themselves to a joint action for some purpose. The third kind of social group is the organization, which is an institution whose members have more or less clearly defined roles, designed to achieve a common purpose. People who join such institutions commit themselves to acting in the ways defined by the institution, fulfilling whatever role they have assumed in accordance with its rules. Our present concern is with the actions of groups of the first two kinds, that is with the actions of aggregates of individuals, and with the behaviour of more or less co-ordinated social groups. The behaviour of organizations is certainly of interest to historians, but beyond the scope of this work.
