The question to be considered here is this: what standards or heuristics ought to govern the choice and definition of units of comparison? Would it make sense to compare World War II with the war in Bosnia? Or the scientific cultures of Bologna in 1400 with that of London in 1960? Or the Spanish Civil War with the culture clash of the United States in the 1960s? What factors make for a historically insightful comparison?
Part of an answer is obvious at the start: there is no hard-and-fast answer to the question. It is always possible that there is a basis of comparison across apparently radically dissimilar cases. So the Spanish Civil War comparison with Madison SDS activism might seem to single out two cases that are too incommensurable to be valuable -- until a creative historian notices that each historical moment revealed deeply different political and moral worldviews and that these ideological differences led to substantial inter-group hostility and conflict in each case. And this comparison might then lead this historian to ask the productive question: why did one instance result in years of warfare and the second resolved into peaceful protest?
Further, it is evident that comparison depends upon the background set of questions that we want to answer; useful comparison is topic-dependent. Comparing Bologna 1400 with London 1960 might be useful if we are investigating the cultural transmission of ideas but useless if we are investigating the causes of urban unrest. And comparing Paris 2006 with Detroit 1967 might be valuable on the second topic but beside the point for the first.
Historically insightful comparison also requires that we not be dazzled by the fact that similar language is used to describe or categorize multiple events. The Chinese Revolution and the Russian Revolution might be described in very similar terms: revolutions resulting from the overthrow of a pre-modern state by a militant party of revolutionaries leading an emerging under-class population. But this description is misleading on its face and in what it conceals. These historical events were actually very different in their political composition, the role and behavior of parties, and the relations that existed among other social forces. The Russian Revolution might be re-described as an opportunistic seizure of power by a minor political party, and the Chinese Revolution might be described as a long, slow mobilization of a mass population in support of revolutionary change. Other descriptions are possible as well. The point is that the common label of "revolution" should not mask the possibility or likelihood of extreme differences in processes, politics, parties, and mobilizations in the two instances.
To get some guidance, we need first to reflect on what the goal of the comparison is. And there are numerous goals that a historian might have. For example --
- discover "generalizations" about similar processes. ("Each of the revolutions studied contains "state crisis" as a causally necessary factor.")
- identify concrete historical mechanisms that are at work in the cases -- whether or not they recur in other cases as well. ("Political mobilization in the first case proceeds through pre-existing religious organizations on the ground; in the second case it proceeds through control of the national media.")
- identify causally salient differences across cases that explain divergence of outcomes. (Here the idea would be an argument something like this: cases A and B are similar in many ways. However, A leads to X, while B leads to Y. What explains this divergence of outcome?)
- discover some of the substantial variety that exists underneath the surface in events that seem superficially similar. (We might pursue this "difference" strategy in urban history and choose a set of examples that will illustrate the many ways in which the world's cities have evolved and are organized and governed.)
Thus the selection of cases will depend on what the purpose of the comparison is. But in general, the only reason to engage in comparison is the likelihood that we will learn something from the comparison that we would not have learned from study of one of the individual cases.
All this said -- we might consider the heuristic that says that the cases need to be similar enough to permit comparison in terms of structures, processes, and causes; different enough to invite inquiry about the causes of the differences; and integrated enough to allow us to say that this level of unit of analysis possesses the complex set of historical characteristcs under study as a whole.
Let's flesh this heuristic prescription out by considering one specific historical question: How did "European economic development" compare with "Asian economic development"? Before we can begin to try to answer this question, we need to give a more thoughtful definition of the units of comparison. Should the comparison be between continents, between countries, between regions within countries, or between selected cities and villages? Kenneth Pomeranz and others argue that continents and nations are too large to serve as a basis of valuable economic comparison, in a very specific sense: they encompass too much variety of social and economic processes to permit valid comparison. Pomeranz argues instead that Eurasian comparison is most valid if we select integrated economic regions of roughly comparable size; large enough to encompass the range of economic, social, and political arrangements that plausibly influence economic development, but not so large that the scale obliterates distinctive patterns and outcomes. Based on these sorts of considerations, he argues that it is most useful to consider a comparison between the core economic regions of England and the Lower Yangzi Delta in China. (James Lee offers an even more disaggregated basis for comparison of demographic regimes; he argues for a comparison based on descriptions of populations at the community level, ignoring nations and large economic regions altogether; Life Under Pressure: Mortality and Living Standards in Europe and Asia, 1700-1900.)
(See Eurasian Historical Comparisons for more on one aspect of this issue.)