Analysis of the dynamic properties of two or more interrelated systems is a recurrent problem in social science theory. A particular problem frequently arises (although it often goes unrecognized) in assessing the validity of an analysis of a system, some variables of which are causally related to other variables, which latter, in turn, are either not explicitly taken into account or are assumed constant. Examples are easy to find: Economists may study the behavior of a single country's economy with only secondary regard for the rest of the world; studies of group behavior may pay only secondary attention to the other roles played by the group members in other contexts; two more examples are worked out below, and others may be found in the works about to be cited. Indeed, in a larger sense, the division of social science itself (or of natural science, for that matter) into separate disciplines is an example, for the variables taken as given by one discipline are the very subject matter of another and vice versa. In all these examples, the very real problem is present that if variables taken as given are causally affected by the variables of the system being analyzed, or if variables assumed not to affect that system 190actually do affect it, the results of the analysis may have little relevance for the study of real problems.