国際政治学における実証分析とマルチエージェント・シミュレーションの架橋

国際社会の基本的規範の交代をめぐって

抄録

Multi-agent simulation (MAS), or agent-based simulation, is very powerful in representing and analyzing system's emergent properties based on interactions among agents. The emergence of social order or norms based on interactions among nations has been one of the central subjects of International Relations (IR). Mutual compatibility being taken into account, MAS could have been applied to IR more extensively. In reality, technical difficulties and excessive abstraction have been most formidable obstacles for the application of MAS. Aiming at a wider use of MAS in IR, we developed a user-friendly simulator, and have built various types of models, from the crisis decision-making to the balance of power, with lesser abstraction and more substantial attributes that represent characteristics of international society.
In this paper, we focus on the spread of an international norm in the manner of cascade on the tipping point, which constructivists of IR such as Finnemore and Sikkink (1998) tried to understand. To date, discussions on this subject have been very suggestive, but far from conclusive. In order to give a theoretical account for norm emergence and cascade, we construct the Norm Emergence Model (NEM). NEM is the combination of the threshold model originally developed by Granovetter (1978) with the persuasion mechanism suggested by Risse (2000) and other constructivists, so that NEM can generate cascades of norm emergence based on dynamic multi-agent interactions that affect the threshold of individual agents.
The case we analyze by NEM is the spread of anti-colonialism that affected the membership of intemational society in the mid 20th century, probably the most fundamental norm-change in international society as Philpott (2001) and others argue. One of the most illustrative events must be the United Nations Declaration of Granting Independence to Colonial Countries and Peoples in 1960. Within less than a generation, the norm of trusteeship was completely replaced by the norm of unconditional independence.
NEM represents virtual international society that sharply split over whether to accept the colonial rules as trusteeship or reject them unconditionally, with the initial condition (agents as UN members, their thresholds and their positions) that more or less reflects the reality at the end of the 1940s at the United Nations. The simulation begins with the year 1950, and finishes in 1969, during which each agent decides which position it should take, and persuades other members aiming at gaining more support of its position. New agents join in the same manner as the reality.
The result of MAS depends on the setting of parameters, and we show results based on four scenarios. Scenario 1 makes persuasion impossible, which means only the entry of new members affects norm emergence. The cascade did not take place. Although ex-colonial countries increase in number, their positions were not as radical as socialist countries. Scenario 2 allows persuasion. Norm change did not happen either. For, socialist countries are persuaded, and change their position so as to accept trusteeship. Scenario 3 allows persuasion, but only anti-colonial norm is influential; trusteeship lost legitimacy. Now, the cascade happens in the mid 1950s with much more sudden pace than the reality. In addition to the setting of Scenario 3, Scenario 4 limits the range of successful persuasion so that agents are affected only when the distance of thresholds is close enough. The cascade takes place in the late 1950s that looks like the reality. Thus, the specific type of interactions and the entry of new members turn out to generate a norm cascade, sometimes after the confrontation for a few years, which seems to reproduce what happened in the arena of the United Nations.
NEM is a very simple model, but can reproduce a cascade of international norm from trusteeship to unconditional independence to a successful degree. ...