It seems at first to be a puzzling scenario, and you might wonder whether it is possible at all: output can be at potential but still not be sustainable. Yet a chapter of the International Monetary Fund’s latest World Economic Outlook illuminates just this scenario. We may even be living in it.
乍看之下，这似乎是个令人费解的情景，你甚至可能怀疑到底有没有可能出现这种情景：产出既是潜在的，却也是不可持续的。然而，国际货币基金组织(IMF)的最新一期《世界经济展望》(World Economic Outlook)里，就有一章阐述了这种情景。我们甚至可能已经身处其中。
Output is “at potential” when it does not generate inflationary or deflationary pressure. Sustainability — and I am referring here to financial sustainability, not the environmental kind — is something else entirely. Output is financially sustainable when spending patterns and the distribution of income are such that the fruit of economic activity can be absorbed without creating dangerous imbalances in the financial system. It is unsustainable if generating enough demand to absorb the output of the economy requires too much borrowing, real rates of interest rates that are far below zero, or both.
当产出不会产生通胀或者通缩的压力，它就是“潜在的”。可持续性——这里我指的是金融方面的可持续性，而不是环境方面的——则完全是另一回事。当支出模式和收入分配使经济活动的成果能被吸收，而不引起金融系统的危险失衡，产出在金融方面就是可持续的。如果产生足够需求来吸收经济中的产出需要过多借贷，或远低于零的实际利率，或者二者兼有，产出就是不可持续的。
To see how that predicament might arise, start by imagining an economy that is balanced in the sense that the amount of money which households and businesses wish to save is exactly the same as the amount they wished to spend on physical investments. So far, so good. But suppose growth of potential output then fell sharply. The level of desired investment would also fall, because the needed capital stock would be smaller. But the amount that people wished to save might not fall, or not by as much; in fact, if people expect to be poorer in future, they might even wish to save more. If so, real interest rates might need to decline sharply, to restore balance between investment and savings.
要理解上述令人费解的情景如何可能产生，可以想象一个平衡的经济体，其中家庭和企业希望储蓄的钱，与希望进行实体投资的钱在金额上完全相等。到目前为止，一切都很好。但设想一下潜在产出增速开始大幅下滑的情况。投资意愿会减弱，因为所需的资本存量变小了。但人们想要储蓄的金额可能不会下降，或者至少不会下降那么多；事实上，如果人们预期自己在未来会变得更加贫穷，他们甚至会希望储蓄更多。如果是这样，实际利率可能需要大幅下降，才能恢复投资和储蓄之间的平衡。
Such a decline in real interest rates might also trigger a rise in the price of long-term assets and an associated surge in credit. These effects would offer a temporary remedy to the faltering demand. But if the credit boom later collapsed, leaving borrowers struggling to refinance debt, demand would then operate under a double burden. The medium-term consequences of excess debt and a risk-averse financial sector would aggravate the longer-term consequences of the weaker potential growth.
实际利率的这种下降还可能引发长期资产价格的上涨，以及信贷的相应激增。这些影响会对需求减弱起到临时的补救作用。但如果之后信贷繁荣崩溃，迫使借款人艰难地对债务进行再融资，那时需求就不得不面临双重负担。过度负债和规避风险的金融业产生的中期影响，会加剧潜在增长减弱引发的长期影响。
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