The topic of this monograph is the social cohesion and incohesion of groups and group-like social systems within the context of the traumatogenic process, in terms of a theory of the fourth basic assumption, which I call ?Incohesion: Aggregation/Massification? or (ba) ?I:A/M?. The theory is illustrated with brief examples from traumatised societies and organisations of various kinds, and with clinical data from group analysis. This work is based on my view that the relationship between the individual and the group is systemic, recursive and kaleidoscopic (Hopper 1982a). A deeper view of persons and groups is not inconsistent with a wider view: it is necessary to think in terms of ?horizontal depth?. In otherwords, the fourth basic assumption in the unconscious life of social systems must always be contextualised in time and space (Hopper 2003). In the context of psychoanalysis, group analysis and the social sciences, it is important to remember that societies and their geographical parts, like cities, towns and villages, and organisations and families, are not actually groups. These various social formations have their own structures and dynamics. Size is not the sole criterion for distinguishing among various kinds of social system. Although inferences from the study of groups to more complex social systems, and vice versa, may illuminate certain properties and processes, the value of this information depends on the degree to which social systems are isomorphic in structure and kind. Indiscriminate comparison hides a variety of assumptions about human nature that consciously and unconsciously influence the kinds of data that are used and the kinds of conclusions that are drawn. However, under certain conditions inferences can be made, and may be especially relevant to the study of social cohesion.