NUCLEUS AND SHIELD - THE EVOLUTION OF SOCIAL-STRUCTURE IN THE ITERATED PRISONERS-DILEMMA

One of the basic questions of social science theory is whether social order can evolve in a Hobbesian world. This question is approached usi ng a micro-based simulation that explains macro-outcomes and can handl e the macrolevel's ensuing effects on micro-decisions. In this way, th e analysis is also an argument for a viable reductionism in the micro/ macro debate. The Hobbesian problem is simulated by an artificial worl d of boundedly rational, innovative individuals playing a multitude of Iterated Prisoner's Dilemmas. Over time, individuals change their str ategies to optimize against everyone else's choices. This simple dynam ic approach reveals a surprising efficiency: Even with substantial amo unts of misunderstanding, cooperation can emerge and be sustained. Mor eover the level of cooperation is high-generally much better than woul d be obtained using TIT FOR TAT The uncoerced cooperation that evolves is stable because it is composed of many different strategies-some ni ce (in the nucleus) and some more cautious (in a protective shield)-ma king it difficult for any contender to beat everyone. Such a dynamic m odel provides a powerful metaphor for a pluralistic society.