In the United States, Europe, Japan, and other 
	developed economies, the recent growth acceleration has been driven by 
	an increase in aggregate demand, a result of continued expansionary monetary 
	and fiscal policies, as well as higher business and consumer confidence. 
	That confidence has been driven by a decline in 
	financial and economic risk, together with the 
	containment of geopolitical risks, which, as a result, have so far 
	had little impact on economies and markets.
	
	Because stronger demand means less slack in 
	product and labor markets, the recent growth acceleration in the advanced 
	economies would be expected to bring with it a pickup 
	in inflation. Yet core inflation has fallen in the US this year and 
	remains stubbornly low in Europe and Japan. This creates a dilemma for major 
	central banks – beginning with the US Federal Reserve and the European 
	Central Bank – attempting to phase out unconventional monetary policies: 
	they have secured higher growth, but are still not 
	hitting their target of a 2% annual inflation rate.
	
	One possible explanation for the mysterious 
	combination of stronger growth and low inflation is that, in addition to 
	stronger aggregate demand, developed economies have been experiencing
	positive supply shocks.
	
	Such shocks may come in many forms.
	Globalization keeps cheap 
	goods and services flowing from China and other emerging markets.
	Weaker unions and workers’ reduced bargaining power 
	have flattened out the Phillips curve(インフレーションと失業の関係を示したもの), with
	low structural unemployment producing little wage 
	inflation. Oil and commodity prices are low 
	or declining. And technological innovations, 
	starting with a new Internet revolution, are reducing the costs of goods and 
	services.
	
	Standard economic theory suggests that the 
	correct monetary-policy response to such positive supply shocks depends on 
	their persistence. If a shock is temporary, central banks should not react 
	to it; they should normalize monetary policy, because eventually the shock 
	will wear off naturally and, with tighter product and labor markets, 
	inflation will rise. If, however, the shock is 
	permanent, central banks should ease monetary conditions; otherwise, they 
	will never be able to reach their inflation target.
	
	This is not news to central banks. The Fed has 
	justified its decision to start
	
	normalizing rates, despite below-target core inflation, by arguing that 
	the inflation-weakening supply-side shocks are temporary. Likewise, the ECB 
	is preparing to taper its bond purchases in 2018, under the assumption that 
	inflation will rise in due course.
	
	If policymakers are incorrect in assuming that 
	the positive supply shocks holding down inflation are temporary, policy 
	normalization may be the wrong approach, and unconventional policies should 
	be sustained for longer. But it may also mean the opposite: 
	if the shocks are permanent or more persistent than expected, normalization 
	must be pursued even more quickly, because we have already reached 
	a “new normal” for inflation.
	
	This is the view taken by the Bank for 
	International Settlements, which argues that it is 
	time to lower the inflation target from 2% to 0% – the rate that can 
	now be expected, given permanent supply shocks. Trying to achieve 2% 
	inflation in a context of such shocks, the BIS warns, would lead to 
	excessively easy monetary policies, which would put upward pressure on 
	prices of risk assets, and, ultimately, inflate dangerous bubbles. According 
	to this logic, central banks should
	
	normalize policy sooner, and at a faster pace, to prevent another 
	financial crisis.
	
	Most advanced-country 
	central banks don’t agree with the BIS. They believe that, should 
	asset-price inflation emerge, it can be contained with macroprudential 
	credit policies, rather than monetary policy.
	
	Of course, advanced-country central banks hope 
	such asset inflation won’t appear at all, because inflation is being 
	suppressed by temporary supply shocks, and thus will increase as soon as 
	product and labor markets tighten. But, faced with the possibility that 
	today’s low inflation may be caused by permanent supply shocks, they are 
	also unwilling to ease more now.
	
	So, even though central banks
	aren’t willing to give up on their formal 2% inflation 
	target, they are willing to prolong the timeline for achieving it, as 
	they have already done time and again, effectively conceding that inflation 
	may stay low for longer. Otherwise, they would need to sustain for much 
	longer their unconventional monetary policies, including quantitative easing 
	and negative policy rates – an approach with which most central banks (with 
	the possible exception of the Bank of Japan) are not comfortable.
	
	This central bank patience risks de-anchoring 
	inflation expectations downward. But continuing for much longer with 
	unconventional monetary policies also carries the risk of undesirable 
	asset-price inflation, excessive credit growth, and bubbles. As long as 
	uncertainty over the causes of low inflation remains, central banks will 
	have to balance these competing risks.
 
NEW YORK – Since the summer of 2016, the global economy has been in a period of moderate expansion, with the growth rate accelerating gradually. What has not picked up, at least in the advanced economies, is inflation. The question is why.