Populations and groups are inherently diverse; virtually any property that might be attached to an individual shows variance across the group. So we have to pay special attention to specifying what we mean when we ask for a "measurement" of a property of a group. This is the basic ontological fact that undergirds a critical approach to quantitative social and behavioral science. And it means that we need always to be considering the variance within the group with respect to the property, the shape of the distribution, as well as the mean value of the property.
It turns out that social phenomena are heterogeneous at the level of institutions, mentalities, practices, and causes as well. Later posts will consider other forms of social heterogeneity. The topic here is institutional heterogeneity. An institution is a system of rules through which a set of social behaviors are mediated. Rules may be enforced through clear third-party enforcement powers (formal institutions) or diffuse participant enforcement practices (informal institutions). Examples of institutions include contract law (formal), cooperative labor-sharing (informal), marriage systems (formal and informal), and tenure systems (formal). Institutions are embodied in the beliefs, values, attitudes, and motivations of socially constructed individuals at various levels of action; they act to constrain and incentivize individual behavior in ways that are to some extent independent of the actions and preferences of those individuals. (That is, the individual is rarely in a position to directly change the rules of the institution so as to serve his/her goals better.) So institutions are both caused by (embodied in) the social consciousness of an extended set of social actors, and are causal in shaping the future behavior of an extended set of social actors.
Institutions have origins -- they come into being at a time and place. So we can ask the question, "what explains the fact of the emergence of the institution and the particular characteristics it possesses at that point?" And institutions undergo processes of development over time -- they undergo change in some characteristics, incorporate new scope and function, and gain new coalitions of supporters and opponents. So we can ask the question, "what factors explain the processes of change that the institution undergoes?" (Kathleen Thelen's How Institutions Evolve: The Political Economy of Skills in Germany, Britain, the United States, and Japan provides a very good account of the ways in which we need to investigate the origins and development of various important social institutions.)
Institutions are sometimes grouped together into broad categories or classes in terms of social function (what does the institution do?), observable characteristics (what does the institution look like?), and social functioning (how does the institution work?). So, for example, we might want to study institutions of marriage-partner selection, irrigation management, or institutions that regulate common property resources such as forests or wetlands; Governing the Commons: The Evolution of Institutions for Collective Action. In each case the group of institutions is defined in terms of the common social problem that they solve.
Now we can frame the task of one important area of sociology and political science research: to undertake careful comparative research concerning the instances of institutions included within a category. In what ways are different examples similar and different from each other? How have parallel or divergent institutional complexes emerged to solve broadly similar social problems? What causal processes can be observed in the workings of these several examples? How do these institutional matrices influence and constrain the forms of behavior that flow through them? (So, in the book by Kathleen Thelen mentioned above, the author considers the national-level institutions of skilled-labor training that have evolved in Germany, UK, USA, and Japan; she considers the effects that these different regimes have on the flow of skilled workers; and she analyzes the political coalitions that were relevant in establishing a particular configuration of the institution.)
Here, finally, we can address the issue of institutional heterogeneity. Given the ways that institutions are formed, changed, and embodied, we should expect that there will be two forms of diversity among institutions. First, it is clear that there are normally multiple ways of solving a particular social problem (training workers for industry, managing prisoners, administering social welfare subsidies). So we should expect that there will be a range of institutional matrices that have emerged across societies to handle these challenges, and we can learn quite a bit about social causation by examining these differences and how they work. And second, given that institutions are "malleable" and dynamic, we should expect that institutions will show diversity within their own life courses. As powerful agents and coalitions shift in their powers and needs, as other constituents acquire more or less influence in setting the agenda for the institution, we should expect an ongoing process of modification of the institution over time.