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The authors discuss how scholars can bring theories of spatial policy dependence and empirical model specifications closer in line so that the empirical analysis actually tests the theoretical predictions. Comprehensive theories of spatial policy dependence typically suggest that the jurisdictions receiving spatial stimuli systematically differ in their exposure to such signals as a function of the intensity of their interaction with other jurisdictions. Similarly, theories often predict that governments also differ in their responsiveness to any given spatial stimulus as a function of the institutional, political, economic, or social context in which they operate. In other words, theories typically postulate that spatial dependence is conditional on exposure and responsiveness, neither of which is accounted for in the standard empirical practice of estimating one single common coefficient for a row-standardized spatial lag variable. The authors show how scholars can adequately model both forms of heterogeneity with properly specified interaction effects models.

Americans are a major target of international terrorism. Yet, terrorists from some countries are much more likely to attack American citizens than terrorists from other countries. Similarly, anti-American terrorism from a specific foreign country is much more prevalent during certain periods than others. This article develops a rational theory of international terrorism, which argues that attacking foreign nationals is of strategic value to terrorists even if they ultimately aim at gaining political influence in their home country. Attacking foreigners is the more attractive to domestic terrorists the more the terrorists' home government depends on military support from the foreign country. Applied to the US case, our theory predicts that more anti-American terrorism emanates from countries that receive more US military aid and arms transfers and in which more American military personnel are stationed, all relative to the country's own military capacity. Estimations from a directed country dyad sample over the period 1978 to 2005 support the predictions of our theory for both terrorist incidents involving Americans and terrorist killings of Americans as dependent variables. These results are robust to a wide range of changes to the empirical research design.

If terror attacks from groups of one country are followed by similar attacks on the same target from groups of other similar countries, then this could be the consequence of contagion. However, just because one terror incident follows another does not necessarily imply that one is caused by the other or, in other words, that terror attacks are what is called spatially dependent. Rather, both incidents could have been triggered by the same underlying cause. This is known as Galton's problem. One area where this problem is particularly prevalent is international terrorism. According to Huntington, international terrorism is contagious because of civilizational rallying effects. If radical groups from one country attack targets from a country of another civilization, then groups from other countries of the same civilization as the initial terrorist groups will become more likely to also attack this target. Any test of this hypothesis has to solve Galton's problem and thus to disentangle spatial dependence from spatial clustering of attacks and common shocks and trends, which affect similar groups from different countries similarly. Accounting for such potentially confounding effects, we nevertheless find evidence for spatial dependence in international terrorism along civilizational lines in the post-Cold War period and particularly so for specific inter-civilizational combinations. However, while contagion consistent with Huntington's predictions exists, spatial dependence seems to have a substantively small effect on patterns of international terrorism.

AbstractThe recent surge in studies analysing spatial dependence in political science has gone hand‐in‐hand with increased attention paid to the choice of estimation technique. In comparison, specification choice has been relatively neglected, even though it leads to equally, if not more, serious inference problems. In this article four specification issues are analysed. It is argued that to avoid biased estimates of the spatial effects, researchers need to consider carefully how to model temporal dynamics, common trends and common shocks, as well as how to account for spatial clustering and unobserved spatial heterogeneity. The remaining two specification issues relate to the weighting matrix employed for the creation of spatial effects: whether it should be row‐standardised and what functional form to choose for this matrix. The importance of these specification issues is demonstrated by replicating Hays' model of spatial dependence in international capital tax rate competition. Seemingly small changes to model specification have major impacts on the spatial effect estimates. It is recommended that spatial analysts develop their theories of spatial dependencies further to provide more guidance on the specification of the estimation model. In the absence of sufficiently developed theories, the robustness of results to specification changes needs to be demonstrated.

AbstractPolitical units often spatially depend in their policy choices on other units. This also holds in dyadic settings where, as in much of international relations research, analysis focuses on the interaction or relation between a pair or dyad of two political units. Yet, with few exceptions, social scientists have analyzed contagion in monadic datasets only, consisting of individual political units. This article categorizes all possible forms of spatial effect modeling in both undirected and directed dyadic data, where it is possible to distinguish the source and the target of interaction (for example, exporter/importer, aggressor/victim, and so on). This approach enables scholars to formulate and test novel mechanisms of contagion, thus ideally paving the way for studies analyzing spatial dependence between dyads of political units. To illustrate the modeling flexibility gained from an understanding of the full set of specification options for spatial effects in dyadic data, we examine the diffusion of bilateral investment treaties between developed and developing countries, building and extending on Elkins, Guzman, and Simmons's 2006 study. However, we come to different conclusions about the channels through which bilateral investment treaties diffuse. Rather than a capital-importing country being influenced by the total number of BITs signed by other capital importers, as modeled in their original article, we find that a capital-importing country is more likely to sign a BIT with a capital exporter only if other competing capital importers have signed BITs with this very same capital exporter. Similarly, other capital exporters' BITs with a specific capital importer influence an exporter's incentive to agree on a BIT with the very same capital importer.

The polity2 variable from the Polity IV project is the most popular measure of a country's political regime. This article contends that the coding rules employed to create a polity2 score during years of so-called interregnum and affected transitions produce a measure of democracy that lacks face validity. Using both single and multiple imputation methods, we construct and evaluate several variables that offer alternative measures to polity2 during such periods. We recommend that scholars using polity2 test whether their results are robust to using our alternatives and using multiple imputation techniques instead. Where robustness cannot be established, scholars need to theoretically justify the choice of either polity2 or one of the alternatives.

AbstractTerrorism is an instrument for groups that cannot achieve their political goals legally. One important strategic function of terrorism is to weaken the government – either directly by attacking representatives or supporters of the government or indirectly by causing a political response that is unpopular among the population. Often, however, political stability of the home government is buttressed by foreign powers. In this case, the terrorists can have a strategic interest in attacking nationals of these foreign countries. This article analyses this logic by looking at international alliances as a proxy for international support. If the friend of my enemy is my enemy, then terror entrepreneurs, which seek to overthrow their home country's government (the enemy), may find it attractive to target nationals of the foreign allies of their country (the friends of the enemy). The theory in this article predicts that attacking nationals of a foreign ally is particularly attractive if this ally is militarily more powerful than the home country. Moreover, the combined effect of alliance and relative power differentials becomes stronger the more democratic the ally and becomes weaker the more democratic the terrorists' home country. Empirical support for the hypotheses in this article is found in an analysis of a directed country dyad sample of international terrorism.