I think heterogeneity is a very basic characteristic of the domain of the social. And I think this makes a big difference for how we should attempt to study the social world "scientifically". What sorts of things am I thinking about here?
Let's start with some semantics. A heterogeneous group of things is the contrary of a homogeneous group, and we can define homogeneity as "a group of fundamentally similar units or samples". A homogeneous body may consist of a group of units with identical properties, or it may be a smooth mixture of different things, consisting of a similar composition at many levels of scale. A fruitcake is non-homogeneous, in that distinct volumes may include just cake or a mix of cake and dried cherries, or cake and the occasional walnut. The properties of fruitcake depend on which sample we encounter. A well mixed volume of oil and vinegar, by contrast, is homogeneous in a specific sense: the properties of each sample volume are the same as any other. The basic claim about the heterogeneity of the social comes down to this: at many levels of scale we continue to find a diversity of social things and processes at work. Society is more similar to fruitcake than cheesecake.
Heterogeneity makes a difference because one of the central goals of positivist science is to discover strong regularities among classes of phenomena, and regularities appear to presuppose homogeneity of the things over which the regularities are thought to obtain. So to observe that social phenomena are deeply heterogeneous at many levels of scale, is to cast fundamental doubt on the goal of discovering strong social regularities.
Let's consider some of the forms of heterogeneity that the social world illustrates.
First is the heterogeneity of social causes and influences. Social events are commonly the result of a variety of different kinds of causes that come together in highly contingent conjunctions. A revolution may be caused by a protracted drought, a harsh system of land tenure, a new ideology of peasant solidarity, a communications system that conveys messages to the rural poor, and an unexpected spar within the rulers -- all coming together at a moment in time. And this range of causal factors, in turn, shows up in the background of a very heterogeneous set of effects. (A transportation network, for example, may play a causal role in the occurrence of an epidemic, the spread of radical ideas, and a long, slow process of urban settlement.) The causes of an event are a mixed group of dissimilar influences with different dynamics and temporalities, and the effects of a given causal factor are also a mixed and dissimilar group.
Second is the heterogeneity that can be discovered within social categories of things -- cities, religions, electoral democracies, social movements. Think of the diversity within Islam documented so well by Clifford Geertz (Islam Observed: Religious Development in Morocco and Indonesia); the diversity at multiple levels that exists among great cities like Beijing, New York, Geneva, and Rio (institutions, demography, ethnic groups, economic characteristics, administrative roles, ...); the institutional variety that exists in the electoral democracies of India, France, and Argentina; or the wild diversity across the social movements of the right.
Third is the heterogeneity that can be discovered across and within social groups. It is not the case that all Kansans think alike -- and this is true for whatever descriptors we might choose in order to achieve greater homogeneity (evangelical Kansans, urban evangelical Kansans, ...). There are always interesting gradients within any social group. Likewise, there is great variation in the nature of ordinary, lived experience -- for middle-class French families celebrating quatorze Juillet, for Californians celebrating July 4, and for Brazilians enjoying Dia da Independência on September 7.
A fourth form of heterogeneity takes us within the agent herself, when we note the variety of motives, moral frameworks, emotions, and modes of agency on the basis of which people act. This is one of the weaknesses of doctrinaire rational choice theory or dogmatic Marxism, the analytical assumption of a single dimension of motivation and reasoning. Instead, it is visible that one person acts for a variety of motives at a given time, persons shift their motives over time, and members of groups differ in terms of their motivational structure as well. So there is heterogeneity of motives and agency within the agent.
These dimensions of heterogeneity make the point: the social world is an ensemble, a dynamic mixture, and an ongoing interaction of forces, agents, structures, and mentalities. Social outcomes emerge from this heterogeneous and dynamic mixture, and the quest for general laws is deeply quixotic.
Where does the heterogeneity principle take us? It suggests an explanatory strategy: instead of looking for laws of whole categories of events and things, rather than searching for simple answers to questions like "why do revolutions occur?", we might instead look to a "concatenation" strategy. That is, we might simply acknowledge the fact of molar heterogeneity and look instead for some of the different processes and things in play in a given item of interest, and the build up a theory of the whole as a concatenation of the particulars of the parts.
Significantly, this strategy takes us to several fruitful ideas that already have some currency.
First is the idea of looking for microfoundations for observed social processes; (Microfoundations, Methods, and Causation: On the Philosophy of the Social Sciences). Here the idea is that higher-level social processes, causes, and events, need to be placed within the context of an account of the agent-level institutions and circumstances that convey those processes.
Second is the method of causal mechanisms advocated by McAdam, Tarrow, and Tilly, and discussed frequently here (Dynamics of Contention (Cambridge Studies in Contentious Politics)). Put simply, the approach recommends that we explain an outcome as the contingent result of the concatenation of a set of independent causal mechanisms (escalation, intra-group competition, repression, ...).
And third is the theory of "assemblages", recommended by Nick from accursedshare and derived from some of the theories of Gilles Deleuze. (Manuel Delanda describes this theory in A New Philosophy of Society: Assemblage Theory And Social Complexity.)
Each of these ideas gives expression to the important truth of the heterogeneity principle: that social outcomes are the aggregate result of a number of lower-level processes and institutions that give rise to them, and that social outcomes are contingent results of interaction and concatenation of these lower-level processes.