为何眼下实际利率这么低？这一局面还将延续很久吗？如果确实如此（目前看来有这样的趋势），那么影响将十分深远：债务人受益，债权人受损，并且最重要的是，不利于全球需求的活跃。
Why are real interest rates so low? And will they stay this low for long? If they do – as it seems they might – the implications will be profound: good for debtors, bad for creditors and, above all, worrying for the vigour of global demand.
在国际货币基金组织(IMF)最近公布的《世界经济展望》(World Economic Outlook)报告中，有一章关于全球实际利率的分析令人关注。以下就是那一章的最重大发现。
The International Monetary Fund’s latest World Economic Outlook includes a fascinating chapter on global real interest rates. Here are its most significant findings.
首先，全球化促使金融一体化。以前，不同国家的实际利率存在很大差距。现在已不是这样，因为如今各地的利率都对共同的影响因素做出反应。
First, globalisation has integrated finance. There used to be wide variation in real interest rates between different countries. That is no longer the case, since interest rates everywhere now respond to common influences.
其次，自上世纪80年代以来，经通胀调整后的实际利率已下降了很多。10年利率接近于零，短期利率则为负值。但预期实际股本回报率（通过股息率和预期股息增长率估算得出）的下降幅度不如实际利率（见图表）。
Second, real interest rates – which are adjusted for inflation – have declined a long way since the 1980s. Ten-year rates are close to zero while short-term rates are negative. But the expected real return on equity (estimated from the dividend yield plus the expected growth of dividends) has not fallen by as much (see charts).
应如何理解这些变化呢？金融资产的实际回报取决于多个不同因素：人们想要储蓄多少、投资多少？储蓄者更愿意持有何种资产？以及货币政策发生了何种变化？这几个因素并非相互孤立。最重要的是，各国央行肩负着达到某一通胀目标的使命，它们必须通过改变货币政策来回应需求变化。
How is one to understand these developments? The real return on financial assets depends on various factors: how much people want to save and invest; what kind of assets savers prefer to hold; and changes in monetary policy. These are not independent of one another. Above all, central banks charged with hitting an inflation target must respond to shifts in demand by changing their monetary policies.
IMF认为，上世纪80年代到90年代初，货币政策的改变是对实际利率影响最大的因素。上世纪90年代末，财政紧缩成为压低实际利率的主要因素。另一个重要因素，是投资品价格相对消费品价格不断降低。信息技术价格相对下降意味着，眼下情况仍然如此。
The IMF reckons that, in the 1980s and early 1990s, changes in monetary policy were the most powerful influence on real interest rates. In the late 1990s, fiscal tightening became the main force driving down real rates. Another important factor was the falling price of investment goods relative to consumption goods. Falling relative prices of information technology mean this is still true.
但自从上世纪90年代末以来，情况发生了很大变化。在新兴经济体，储蓄率上升，主要是因为收入在增加。投资者开始青睐所谓安全资产。最重要的是，近来的金融危机导致在受到冲击的经济体中，投资显著减少、私人储蓄大幅增加。
Since the late 1990s, however, much has changed. In emerging economies the savings rate has gone up, largely because incomes were rising. Investors began to favour assets deemed safe. Most importantly, recent financial crises have caused investment to collapse and private savings to jump in the affected economies.
IMF提出，通胀风险降低没有起到助力长期利率降低的作用，因为“期限利差”——指短期利率与长期利率之间的差值——并未下降。更重要的是国民储蓄与投资变化的影响。在全球范围来看，储蓄必然等于投资。所以，我们观察到的全球储蓄率，根本无法告诉我们“储蓄过剩”是否一直在升高。此处“储蓄过剩”是指超出意愿投资额的那部分意愿储蓄。唯有价格——实际利率——的变化能揭示这一点。
The IMF argues that declining inflation risk has not contributed to the fall in long-term rates, since the “term spread” – the gap between short and longer-term rates – has not fallen. More important has been the effect of changes in national savings and investment. At the global level, savings must equal investment. So changes in the observed global savings rate will tell us nothing about whether there has been a growing “savings glut” – by which I mean an excess of desired savings over desired investment. Only a shift in the price – the real rate of interest – reveals that.
引人注意的是，上世纪90年代中，10年期实际利率为4%，到本世纪头十年（危机之前）降至2%，再后来降至接近零。10年期实际利率陡然下跌背后至少有两个因素在起作用。高收入经济体投资一路下滑，但新兴经济体——尤其是中国——投资却大举升高；不过，新兴经济体储蓄率增幅甚至比投资率增幅还要高。结果，这些经济体成了资本净出口大国。
Strikingly, the 10-year real rate of interest was 4 per cent in the mid-1990s, 2 per cent in the 2000s, before the crisis, and close to zero thereafter. At least two factors lay behind this precipitous fall. Investment fell a long way in high-income economies but soared in emerging ones, especially China; yet the savings rates of emerging economies rose even more than their investment rates. Consequently, these economies became big net exporters of capital.
新兴国家也将大部分资本输出国有化了。于是，它们的政府倾向于购买“安全”资产，尤其是作为外汇储备。这有助于解释投资组合为何向高评级债券倾斜。
Emerging countries also largely nationalised this capital outflow. Their governments then tended to buy “safe” assets, especially to put in the foreign exchange reserves. This helps explain the portfolio move towards highly rated bonds.
简言之，情况是意愿实际储蓄与意愿实际投资之间的差额发生变化，导致实际利率大幅下降。另外两个因素是，投资组合偏好发生变化、向安全资产倾斜，以及2000年前股票泡沫的破灭。在高收入国家，收入分配向资本方和高薪员工倾斜，也弱化了需求。于是各中央银行推出了激进的货币政策。这推动了信贷大膨胀，信贷大膨胀通常看来与房价猛涨脱不开干系。信贷和房价在危机中都崩溃了。正如劳伦斯•萨默斯(Lawrence Summers)所指出那样，令人担忧的是，高收入经济体似乎无法在不制造极端信贷动荡的前提下，推动需求强劲增长。
The story, in brief, is that shifts in the balance between desired real savings and investment generated a large fall in real interest rates. These were accompanied by changes in portfolio preferences towards safe assets and the collapse in the pre-2000 equity bubble. The shift in the distribution of income towards capital and highly paid employees in high-income countries also weakened demand. The central banks then responded with aggressive monetary policies. These supported explosions of credit generally linked to house-price surges. Both imploded in the crisis. As Lawrence Summers has argued, the high-income economies seem to be worryingly unable to generate good growth in demand without extreme credit instability.
这种情况并不是短期内的。“长期增长停滞”(secular stagnation)看上去是个贴切的论断。IMF赞同实际利率可能在长期内维持低位的观点。如果各国政府坚持按计划实行紧缩的财政政策，则看来注定会出现这种局面。如果中国投资率大幅下降，那么全球实际利率可能需要进一步下降。在通胀如此低的情况下，这很难办到。
This is not a short-term story. The label “secular stagnation” looks apposite. The IMF agrees that real interest rates could remain low for a prolonged time. If governments persist with planned tightening of fiscal policies, this seems certain. If investment rates fell sharply in China, global real rates might need to fall still further. That is difficult while inflation is so low.
靠什么才能扭转这一局面？一个显而易见的可能药方，是相对较高的预期股本收益率驱动高收入国家的投资猛增。这里有三重障碍。一是首席执行官没有动力为长期而投资；另一个是投资品的价格一直在降低；最后一个是，当未来不确定、经济又低迷时，企业出于理性考虑，愿意观望而不愿投资。
What might reverse this? The obvious possibility is a jump in investment in high-income countries driven by the relatively high expected returns on equity. The obstacles here are threefold. One is that chief executives are not rewarded for investing for the long term; another is that investment goods are becoming cheaper all the time; and another is that, when the future is uncertain and the economy sluggish, companies rationally prefer to wait before they invest.
第二个可能药方，是新兴经济体储蓄率大幅下降。但这似乎不可能实现，至少油价不暴跌的话就不太可能实现。那就只剩下一个选择，即高收入国家维持财政赤字，最好投资于基础设施。与房地产相联系的信贷繁荣则是一个糟糕得多的选择。通过再分配向花钱者倾斜看上去不可思议。
Another possibility is a big fall in savings in emerging economies. But this seems unlikely, at least without a collapse in oil prices.** That leaves the option of sustained fiscal deficits in high-income countries, ideally to be invested in infrastructure. Housing-related credit booms are a far worse option. Redistribution towards the spenders seems quite inconceivable.
如果实际利率确实长期维持在低水平，那么债权人未来会觉得日子不好过。但管理后危机时代的公共财政，应当会比歇斯底里者认为的容易得多。如此世界一个真正的大问题，是常规的通胀目标定得可能太低了，因为没有给实际利率在必要时降至负值留下足够大的空间。
If real interest rates do indeed remain low for a long time, creditors are going to find life difficult. But managing the post-crisis public finances should be far easier than the hysterics assume. A really big question in such a world is whether conventional inflation targets might be too low, because they do not give enough room for real interest rates to fall as far below zero as necessary.
不过，当前的紧迫问题是，我们应如何产生支撑潜在全球供应所离不开的需求？不能以合理方式满足这一要求是过去引发危机的一个主要因素。这一要求继续得不到满足将阻碍复苏，更严重者，将引发又一轮金融与经济动荡。不要臆想这些挑战不久后就会消失，它们看来就像一种半永久性的病症。
The immediate question, however is: how do we generate the demand that is needed to mop up potential global supply? Failure to answer that need in a sensible way was a leading cause of the crisis. Continued failure will blight the recovery or, worse, cause another bout of financial and economic upheaval. Do not imagine these challenges will soon vanish. They look like a semi-permanent condition.