Image: Mason-Pfizer monkey virus (link)
Image: organization chart of General Motors
What does it mean to say that the social world is contingent? Several things. First, it means that social changes and patterns are not strongly law governed. Outcomes are the result of intersecting chains of causal mechanisms and stochastic happenings, so there is no sense in which outcomes are predetermined or confidently predictable. Social outcomes are the result of conjunctural causation, with indeterminate conjunctions of causal processes and conditions proceeding from independent background circumstances. And accidents and random events make a difference in the outcomes as well. This is true at a full range of scales, from large happenings like the outbreak of war to the growth of a corporation to the emergence of a new set of values about gay marriage. So historical processes and sequences are contingent, and we need to pay close attention to the path dependency of social happenings.
Another key kind of contingency has to do with the composition of social entities. In the natural world there are some formations that are necessary. H2O and protein molecules have a specific topology and arrangement that follows strictly from the physical properties of the constituents, and these properties, we would like to assert, are fixed by nature. So it is a necessary fact that H2O molecules all have the same topology -- this topology follows from physical laws. But likewise, large proteins have only a small number of stable geometries as well, given the physical characteristics of the atoms that compose them.
The situation is different for social compounds. They are composed of individuals. But their properties are not fixed by the laws of psychology or any other consistent realm. Rather, there is substantial path dependency in the formation of a particular social formation, and the properties of actual social formations are contingent relative to the properties of the individuals who constitute it.
To say that social phenomena are contingent is not to imply that they are random or unpatterned. In fact, a large part of the task of the social sciences is to identify and explain important social patterns -- for example, regularities of urbanization and habitation. G. William Skinner found that the cities, towns, and hamlets of Sichuan conformed to a pattern of nested hexagons (link); he offered the mechanisms associated with central place theory as the basis for an explanation of this fact. The combined workings of transportation cost and cost-sensitive individual decision-makers imply the hexagonal geometry that Skinner discovers. But there is vast contingency embedded within this account -- reasons why certain locations may be avoided or reasons why a given center may come to have higher-level commercial or military functions than would have been expected, for example. So the regularities that we observe can be explained by the workings of several social mechanisms that favor habitation choices; while extraneous factors can disrupt or distort the pattern that would be normally expected.
So there are conditions and influences that often create identifiable patterns of social activity. This is the chief reason why the study of social mechanisms is so fruitful in the social sciences: there is an open-ended plurality of causal mechanisms at work in the social space. These can be investigated and understood. And we can then use our ability to identify the workings of social mechanisms to provide explanations of both singular occurrences and intriguing social patterns. But at the same time, we are forced to recognize that particular social processes -- economic development, urbanization, political crisis, ethnic conflict, or changes in values systems in a population, for example -- are driven by multiple sub-processes that are themselves contingent, and that interact in contingent ways.
The evolution of species as described by classical Darwinian theory is a good example of the some of aspects of contingency that I believe are characteristic of social developments as well. The large pressures within a given ecological environment are those that affect reproduction and longevity. Variations occur within the genetic information constituting organisms at a certain time, and natural selection favors the proliferation of some of those variations into the population as a whole. But the longterm evolution of the X group of organisms is not pre-determined; X's may invest in better vision, better mobility, greater lethality as predators, greater ability to conceal from predators, or dozens of other possible lines of evolutionary change. So all we can predict is that the assortment of groups of organisms will evolve towards higher levels of reproductive fitness or will disappear; and we can explain, in hindsight, the emergence of some of the physiological characteristics of X's in terms of the reproductive advantage that this feature confers on the organism. So there is nothing in the antecedent habitat that preordains that giraffes will have long necks.
There is an important analogy here with social change. We can identify some of the features that influence the development of organizations and political institutions in a variety of historical settings: the need for states to extract revenues and to exert coercive power, for example. But we cannot predict with confidence what form those adaptations will take. So the theatre state of Bali looks very different from the feudal monarchy in France, even though both states succeed in the central functions of states.