Society is a complex, compositional entity. Both persistence and change require explanation in such entities. Because there is a third alternative condition of such an entity: chaos, randomness, and Brownian motion of individuals in interaction with each other. Succinctly -- the current state of the ensemble is simply the sum of the states of the lower-level units, and their states are in turn determined by a stochastic set of encounters with other individuals in the prior period. Both persistence and patterned change are non-random outcomes for a compositional system, and each demands explanation. (It is as if crystals formed periodically in boiling water among the micro-particles observed in the solution.) So if we observe either persistence or patterned change in spite of underlying stochastic processes, there must be causes of these non-random outcomes; so we want to be able to discover the causes of both.
To consider change and persistence, we need to answer a prior question: change and persistence of what? Logically, an ensemble demonstrates change and persistence with respect to some set of characteristics: features of organization, patterns of distribution of lower-level properties (e.g. income, attitudes), characteristics of "meso"-level behavior. In biological terms, the ensemble possesses structures, functions, and dynamics of development. And we need to be able to explain each of these gross features in a way that is consistent with the compositional nature of social entities.
We can easily produce examples of each type of condition in social life.
- Persistence: The Federal Reserve Board retains its institutional form and its functional role within the US economy over a 50-year period. Baseball is still governed by the same basic rules as it was in the nineteenth-century (with minor variations).
- Change: American attitudes about race shift measurably over 50 years. The percentage of workers in labor unions falls dramatically from 1970to 2000.
- Stochastic: The frequency of the name "Harry" falls dramatically from 1950 to 1990. Traffic on Skype fluctuates from minute to minute.
Given that the ensemble is composed of lower-level elements (individuals), we want to know what it is that constrains, impels, and conditions the behavior of the individuals, such that the ensemble comes to have the observed features of persistence and change.
In order to explain either persistence or change in social arrangements, what do we have to work with? Only two sorts of things, fundamentally. We have the behavioral characteristics of the individuals who constitute the society; and we have the behaviorally relevant features that are embodied in current social institutions and practices (rules, incentives, opportunities, forms of social cooperation and social punishment). The latter "social" facts are themselves embodied in the behavioral characteristics of the persons who constitute them; but at any given time they function with apparent autonomy with respect to particular actors.
Fundamentally, then, the explanation of both persistence and change in society requires that we uncover the features of individual motivation that guide their behavior (interests, values, preferences, identities, practices); the features of the current social environment that empower and limit individual strategies; and the processes of aggregation through which the individuals' actions come together into collective behavior in either reinforcing or disrupting the social facts of current interest. This intellectual model for social explanation is what I refer to as "methodological localism."
(See "The Heterogeneous Social" for more on methodological localism.)