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The Diamond coconut model is an economic model constructed by the American economist and 2010 Nobel laureate Peter Diamond which analyzes how a search economy in which traders cannot find partners instantaneously operates. The model was first presented in a 1982 paper published in the ''Journal of Political Economy''. The main implication of the model is that people's expectations as to the level of aggregate activity play a crucial role in actually determining this level of aggregate economic activity. A frequent interpretation of its conclusion, as applied to the labor market, is that the so-called natural rate of unemployment may not be unique (in fact there may exist a continuum of "natural rates") and even if it is unique, it may not be efficient. Diamond's model was of interest to New Keynesian economists who saw it as potential source of coordination failure, which could cause markets to fail to clear.〔"Mankiw, N Gregory and Romer, David. "Introduction." ''New Keynesian Economics''. Vol. 1. 1991.〕 The model takes its name from the abstract set up imagined by Diamond. He envisioned an island (a closed economy) populated by individuals who only consume coconuts. Coconuts are obtained by being picked (they are "produced") from palm trees at a cost. Because of a particular taboo existing on this island a person who has picked a coconut cannot consume it themselves but must find another person with a coconut. At that point the two individuals can trade their respective coconuts and eat them. The key point is that when an individual finds a palm tree, because climbing the tree is costly, they will only be willing to climb it to get a coconut if there are a sufficiently high number of other individuals who are willing to do likewise. If nobody else is obtaining coconuts then there won't be any potential trading partners and obtaining coconuts is not worth climbing the tree. Hence, what individuals believe others will do plays a crucial role in determining the overall outcome. As a result people's (fully rational) expectations become a self-fulfilling prophecy and the economy can wind up with multiple equilibria, most if not all of them characterized by inefficiency. ==Population flows in the model== The agents in the model are always in one of two "states"; they are either currently carrying a coconut and looking for someone to trade it with, or they are searching for a palm tree in order to possibly pick a coconut. The number of agents who are carrying a coconut at time t is denoted by (for "employed") and they find trading partners at the rate at which point they trade coconuts, earn income and become "searchers". The fact that the probability of finding a trading partner is increasing in the number of people who already have coconuts - mathematically - represents a "thick market externality"; the "thicker" the market in the sense of more potential traders, the more trades occur. It involves an externality because each person who chooses to pick a coconut does so with only their own self-interest in mind, but the fact that they do so has an effect on the overall social outcome. People who are currently looking for coconut palm trees find these at a random rate . This means that the finding of palm trees follows a Poisson process characterized by the parameter . If total population is normalized to 1 (hence, is the share of the population that is employed) then the number of searchers in this economy is . The figure above illustrates the population flows in this economy. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Diamond coconut model」の詳細全文を読む スポンサード リンク
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