全球經濟面臨不可持續難題
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，這似乎是個令人費解的情景，你甚至可能懷疑到底有沒有可能出現這種情景：產出既是潛在的，卻也是不可持續的。然而，國際貨幣基金組織(IMF)的最新一期《世界經濟展望》(World Economic Outlook)裏，就有一章闡述了這種情景。我們甚至可能已經身處其中。
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.
當產出不會產生通脹或者通縮的壓力，它就是“潛在的”。可持續性——這裏我指的是金融方面的可持續性，而不是環境方面的——則完全是另一回事。當支出模式和收入分配使經濟活動的成果能被吸收，而不引起金融系統的危險失衡，產出在金融方面就是可持續的。如果產生足夠需求來吸收經濟中的產出需要過多借貸，或遠低於零的實際利率，或者二者兼有，產出就是不可持續的。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.
要理解上述令人費解的情景如何可能產生，可以想象一個平衡的經濟體，其中家庭和企業希望儲蓄的錢，與希望進行實體投資的錢在金額上完全相等。到目前爲止，一切都很好。但設想一下潛在產出增速開始大幅下滑的情況。投資意願會減弱，因爲所需的資本存量變小了。但人們想要儲蓄的金額可能不會下降，或者至少不會下降那麼多；事實上，如果人們預期自己在未來會變得更加貧窮，他們甚至會希望儲蓄更多。如果是這樣，實際利率可能需要大幅下降，才能恢復投資和儲蓄之間的平衡。
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.
實際利率的這種下降還可能引發長期資產價格的上漲，以及信貸的相應激增。這些影響會對需求減弱起到臨時的補救作用。但如果之後信貸繁榮崩潰，迫使借款人艱難地對債務進行再融資，那時需求就不得不面臨雙重負擔。過度負債和規避風險的金融業產生的中期影響，會加劇潛在增長減弱引發的長期影響。
