“Zones of peace” and similar political systems
After having closed some research gaps, the results of the case studies leave other gaps open and establish new ones. The main avenues for future research are, first, to intensify the study of SPSP, both within institutionalist and social constructivist frameworks; second, to deepen the study of the specific monarchic variant inside and outside of the provided framework; and third, to address other questions on the margins of this study that have been left unanswered.
First, having provided one building block of the SPSP, future studies should look at other possible cases under even-more-diverse conditions, ideally also in cross-regional comparisona. The "zones of peace” that could form part of the same typology as the MP and the DP mentioned earlier are prime candidates. Studies comparing these “zones of peace” to the MP and DP might flesh out and make the SPSP more robust on the basis of a stronger definition of "political similarity”, which could in turn be operationalized for large-n studies.
Second, since this work focused on the most striking elements of foreign policy - war and peace - further smdies should look at other elements that characterize interaction among states, regimes and rulers, including different forms of cooperation, trade and cultural relations, or membership in international fora. While this book focused on a psychological and social constructivist approach (SIT), further smdies should probe the institutionalist account with other subtypes of the SPSP. SIT and institutional approaches might have an additive effect. For instance, as argued earlier, specific institutional features might make some groups of states more predisposed to develop an ingroup identification than others. While SIT-based approaches claim that peacefill group members are peaceful because they are similar to each other, institutionalist approaches instead state mostly that they behave in a peaceful way because they are similar to democracies. Maybe some subtypes of autocracy, like monarchies or single-party regimes, are indeed more similar institutionally to democracies and therefore behave in a similar way, as previous research suggested (Mattes and Rodríguez 2014; Sirin and Koch 2015; Weeks 2012). In short, although institutionalist arguments have been presented in the theoretical section as an alternative or rivaling explanation, they might in fact be complementary and as such a stepping stone toward further analytical precision.
Institutionalist arguments might also open up the way to examine whether the emergence, preservation, and deepening of an ingroup is shaped by the same drivers. The case studies suggested that a common threat might be more important at the outset of ingroup formation but that it might not be as essential for affirming and deepening the ingroup. There should also be further studies to answer the question under what conditions such ingroups develop. This book has shown how three main factors affect the salience of monarchy (overall share of monarchies.
Conclusion 241 common threat, and divisive ideology), but other groups of states might work under slightly different conditions.
Third, further questions concern more-detailed aspects of the causal mechanism that arose over course of the investigation. For one, what is the relative importance of regime type vis-à-vis other dimensions of similarity? For instance, the attempted integration of the ties of some core groups of monarchies in the case studies (especially the GCC and AFU) indicated that political system similarity goes a long way to explain non-war, but for deepening alliance and integration, cultural or linguistic similarity might be necessary. This points to an additive relationship between the different dimensions of similarity, but the question remains whether and under what conditions one could supplant the other. This aspect is also linked to the question of how exactly some similarities relate to regime type, like worldview, a conservative mindset, or a particular set of preferences. The argument that more similarity apart from regime type leads to a stronger ingroup might not be surprising, but it confirms that instead of Policy IV values, the study of “similarity” needs to be grounded in much-deeper analyses of history and culture.
Another question concerns the tipping point that determines when exactly a community of peace becomes a community of active solidarity or an institutionalized alliance. The case studies hinted at but did not systematically examine that this process might be shaped by two factors: the intensity of similarity (including more dimensions of similarity apart from the political system) and the intensity of a common threat.