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.

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。

The International Monetary Fund’s latest World Economic Outlook includes a fascinating chapter on global real interest rates. Here are its most significant findings.

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。

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.

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。

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).

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）。

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.

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。

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.

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。

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.

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。

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.

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。

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.

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。

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.

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。