Federal Reserve Chair Janet Yellen Saul Loeb/Getty Images

中央银行合方能利,不合必败

纽约—全球经济正在朝复苏的方向走,尽管缓慢,但非常确定。国际货币基金组织(IMF)的最新世界经济展望预测今年全球增长为3.5%,较去年的3.2%有所提高。但有一个问题:让经济回归增长的宽松货币政策正在达到极限,现在,它们有可能在为新的金融危机创造条件,对复苏起到干扰作用。

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近几年来,世界主要央行采取了前所未有的宽松货币政策,包括德意志银行最新报告中所谓的“几百年来最低水平的利率”。这,再加上大规模量化宽松,在过去九年中为全球经济注入了32万亿美元巨资。但这些非常规政策正在转变为经典的博弈论坏均衡:每个央行都能从保持低利率中获利,但从集体角度,它们的方针却构成了一个陷阱。

在当今全球化的世界,个别央行略微调低利率能够带来一些好处,首先是货币贬值,从而提振出口。但采取这一策略的国家越多,银行业的压力就越大。在欧洲,这一点已经表露无遗,欧洲银行股价在最近几个月中稳步下跌。

此外,低利率,特别是负利率,能产生现金持有成本,促使投资者寻求通过高风险投资实现更高的潜在回报。结果,抵押贷款证券(CLO)在今年翻了一番,总市值达到了4,600亿美元。这看起来和导致2008年金融危机的抵押债务证券(CDO)规模飙升如出一辙。尽管世界对CLO的制约要比危机前的CDO更强,但这一趋势仍然令人深感不安。

最后,持续低利率可能导致人们担心自己的退休金,促使他们增加储蓄。货币刺激并不能如愿提振消费,反而有可能创造一个抑制需求的环境,削弱经济增长前景。

如今,没有一个国家能够让世界摆脱这一陷阱。昔日领袖美国在最近几年中让出了全球领导地位——这一过程在特朗普担任总统的第一年里大大加速了。此外,G20在支持世界主要发达和新兴经济体的货币和财政政策协调方面也日益力不从心。

也许需要在为时未晚之前成立新的主要行动方——G大国(GMajor)。要产生必要的激励,货币决策者应该回顾一下“旅行者悖论”,这个博弈论例子突显了个体理性的缺陷。

这个例子讲的是一群旅行者带着从遥远的小岛上买到的一样的陶器回家。他们发现陶器在运输过程中被损坏了,于是要求航空公司赔偿。航空公司经理——称为“财务高手”——不知道陶器的价格,他需要开动脑筋来确定恰当的赔偿数字。

经理决定让每位旅行者写下价格——介于2美元和100美元的任意整数——但不能彼此通气。如果所有人写的数字一样,那么这个数字就被认定为陶器的价格,也就是每位旅行者获得的赔偿额。如果他们写的数字不一样,那么就以最低数字为正确价格。写下最低数字的人,能够额外获得2美元作为诚实的奖赏,而写的数字超过最低数字的人,将被扣去2美元作为作弊的惩罚。也就是说,如果有人写了80美元,有人写了90美元,那么他们分别将获得82美元和78美元赔偿金。

一开始,旅行者们非常兴奋。陶器没有实际货币价值,但如果他们每个人都写100美元,所有人都能获得100美元年赔偿。但有一位旅行者很快认识到,写99美元是更好的选择,因为这会带来2美元额外奖励,从而让总数达到101美元。但这位旅行者很快又认识到,其他人肯定也有同样的想法,因此他决定把数字降低到98美元。但如果其他人也是这么想的呢?最好还是写97美元吧。

最后,陷在这个无懈可击的逻辑中的旅行者,每个人都写下并获得了2美元。结果看上去是一场灾难,但也是最理性的选择——即旅行者悖论博弈的“纳什均衡”。很显然,财务高手名副其实。

这个故事的道德含义很简单。市场的看不见的手并不总是能让个体的自利动因转化为集体渴望的结果。利他主义和顾及他人必须起到作用。如果没了它们,行动者至少需要协调他们的决定。除非央行认真汲取了这一教训,否则它们会发现它们得打扫无数陶器碎片。

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