Daniel Posner
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* African Borders as Sources of Natural Experiments Daniel N. Posner Department of Political Science University of California, Los Angeles Los Angeles, CA 90095-1472 [email protected] This version: 12 February 2006 * Paper prepared for presentation in the Colloquium Series of the Yale Program in Agrarian Studies, 17 February 2006. I thank John McCauley for sharing his data on religious identification along the Côte d’Ivoire-Burkina Faso border and Anoop Sarbahi for his help assembling the GPCP rainfall data. The arbitrary nature of Africa’s borders is well known.1 Less well recognized is the opportunity that these arbitrary borders provide for social science research. This paper describes some of the shortcomings of traditional, observational studies and shows how natural experiments can provide for more reliable inferences about causality. The paper then draws on examples from three different projects to illustrate how the artificial nature of Africa’s borders can be used as a source of natural experiments. The Quest for Causal Inferences in Social Science Research In the social sciences, researchers typically make causal inferences through observational studies. These are analyses where the variables that the researcher is studying acquire their values through the unfolding of real-world events that are outside the researcher’s control (Collier et al 2004). The problem with observational studies is that they make it difficult or impossible to rule out the possibility that other, unmeasured variables may be the cause of the observed relationship between the variables of interest. Suppose, for example, we are interested in the linkage between smoking and cancer. To investigate the connection, we might recruit a large number of smokers and non-smokers and compare their cancer rates. If we found higher cancer rates among the smokers, we might be tempted to conclude that smoking does indeed cause cancer. But we would be wrong to reach such a conclusion unless we could first rule out the possibility that other unobserved factors – for example, poverty or stress or diet – might have caused people both to smoke more and to be more susceptible to disease. If we just took smokers and non-smokers “as we found them” – which is what researchers in observational studies do – then we would be unable to rule out these potentially serious competing explanations for the relationship we found between smoking and cancer. This is the weakness of observational studies. They may be useful for identifying possible causal effects, but they cannot reliably establish causation (Gerber et al 2004). Experimental studies, such as those commonly undertaken in the natural sciences, do permit causal inferences to be made. In experimental research, the values on the key independent variables are products of the random assignment of cases by the researcher to different treatments. It is precisely the random assignment of subjects to treatments that permits the researcher to rule out confounding explanations of the sort described above. For social scientists, the major drawback to experimental studies is that they are difficult to undertake outside of the laboratory. Also, experiments done in the lab often have questionable validity in the real world. Nonetheless, precisely because of their advantages in making causal inferences, the use of experimental methods in the social sciences – increasingly through field studies (e.g., experiments that take place outside the laboratory) – is growing (McDermott 2002). Natural experiments stand as a middle ground between observational and experimental studies. Like observational studies, they derive from naturally occurring phenomena rather than the manipulation of the experimenter. But, unlike observational studies, they permit the researcher to “make a credible claim that the assignment of the non-experimental subjects to treatment and control conditions is ‘as if’ random” (Dunning 2005). This means that they 1 For an excellent recent summary of this literature, see Englebert et al (2002). 1 African Borders as Sources of Natural Experiments Daniel N. Posner combine the methodological power of experiments with the “out of the lab” flexibility of observational studies. The, however, trick is to find them, for good ones are as rare as they are valuable. This paper shows how the arbitrary boundaries between African countries can be viewed as a random assignment of populations to different national treatments, and thus as a rich source of natural experiments. African Borders as Sources of Natural Experiments The borders between African countries today are products of colonial era boundary- making. Although sometimes delimited on the basis of language, religion, or other sociological characteristics, Africa’s boundaries were, for the most part, arrived at with no reference at all to the social or cultural characteristics of the people they partitioned. A clear indication of the arbitrary nature of Africa’s borders is the fact that 44% of them follow meridians or parallels and another 30% follow other rectilinear or curved lines. An indication of their disregard for the populations they bisect comes from Asiwaju’s (1985) estimate that Africa’s 104 distinct borders divide 177 cultural or ethnic groups. If we accept the physical location of a given border as arbitrary (that is, exogenous to all the potentially relevant sociological, cultural and historical facts on the ground), then we can treat the division of people on either side of the border as a random assignment of subjects to different treatments – the treatments provided by the different political institutions, country-level demographics, histories, and public policies in effect in the countries on either side of the boundary. This implies that African boundaries can serve as sources of natural experiments. This paper draws on materials from three different projects – one completed, one ongoing, and one just beginning – to illustrate how Africa’s arbitrary borders can generate natural experiments.2 The first project exploits the arbitrary positioning of the Zambia-Malawi border to study the different inter-group relations between identical groups living on either side of the border. From an objective standpoint, the cultural cleavage between these two communities is identical on either side of the border. But it is much more politically and socially salient in one country than the other. The project explains this difference by emphasizing the different sizes of each group in each country relative to each country’s national population, and shows that this has important implications for the usefulness of each group as a basis for political coalition formation, and thus for whether or not the group (and the cleavage between it and its neighbors) is mobilized politically and thereby rendered politically salient. The second project is similar to the first, in that it explores attitudinal and behavioral differences among members of the same population that happens to be bisected by an arbitrarily demarcated border – this time between Burkina Faso and Côte d’Ivoire. But whereas the variation being explained in the first project is in the nature of relations between pairs of groups, the variation being explained in the second is the salience of different social identities within the same (divided) community. Specifically, I explore why religion is more salient to people on one 2 In doing so, I join a number of other authors (e.g., Asiwaju 1985; Laitin 1986; Miguel 1984; Miles 1994, 2005; Miles and Rochefort 1991) who similarly take advantage of the arbitrary placement of country boundaries in Africa as a tool for making causal inferences. 2 African Borders as Sources of Natural Experiments Daniel N. Posner side of the border than the other. I explain this difference by documenting the political mobilization of religious divisions in one country but not the other. The project thus underscores the political sources of religious identification. The third project uses Africa’s arbitrary borders to study how different countries and populations react to identical exogenous climate shocks – specifically droughts. The project uses rainfall data based on satellite images of cold cloud cover to identify droughts that affect neighboring countries (or neighboring regions on either side of a national boundary). Then, using the assignment of drought-affected areas to different countries as a form of random assignment, the project investigates the sources of the different responses to and impacts of the drought on either side of the border. This project is still in its beginning stages, so I explain the set-up and provide some working hypotheses that I plan to explore, but I do not present any results. Why Are Cultural Cleavages Sometimes Politically Salient and Sometimes Not?3 The first project I will describe exploits the division of the Chewa and Tumbuka peoples by the border between Zambia and Malawi. The Zambia-Malawi boundary follows the watershed of a set of low hills that run from southwestern Tanzania to northwestern Mozambique, roughly parallel to the shoreline of Lake Malawi. The boundary was originally demarcated by the British South African Company in 1891 to distinguish the territories of what were then called Northeastern and Northwestern Rhodesia. Like many African borders, it was drawn purely for administrative purposes with no attention to the distribution of groups on the ground. The way it divided the Chewa, Tumbuka, and other communities in the area can thus be treated as a random assignment to two treatments – what might be thought of as the “Zambia treatment” and the “Malawi treatment” – which can be exploited to explore the impact of country-level factors such as political institutions, demographics, leadership styles and public policies on local social and political outcomes. Chewas and Tumbukas living on one side of the border are culturally identical to their co-ethnics living on the other side. Thus, we might expect Chewa-Tumbuka relations to be the same in both countries – particularly if we thought that the nature of the social interactions between two communities was a product of the degree of cultural difference between them.