Can bubbles be identified?
Despite the many problems propagated by bubbles,
some have argued that central banks should ignore
bubbles.Their argument consists of two parts: (1) central
banks cannot reliably determine if a run-up in
stock prices is actually a bubble until after it has burst;
(2) an interest rate hike of sufficient magnitude to
prick a suspected bubble would likely send the economy
into a recession, thereby sacrificing the benefits
of the boom that might otherwise continue.
To identify a bubble in real time, policymakers would
need to judge whether a valuation metric, such as the
price-earnings (P/E) ratio for the aggregate stock market,
has crossed into bubble territory. But rendering
such a judgment may be difficult in practice,particularly
if the bubble is triggered by investor overreaction to
an actual fundamental improvement in the underlying
economy. For example, a pickup in measured productivity
growth during the late 1990s appeared to offer
to some fundamental justification for the rise in the
market P/E ratio. Others have countered that the difficulties
in identifying bubbles do not justify ignoring
them altogether.Central banks routinely apply judgment
to a whole host of issues affecting monetary policy.
One example is the size of the so-called “output gap,”
defined as the difference between actual GDP and
potential GDP. The output gap is notoriously difficult
to measure in real time, yet it remains an important
input to central bank inflation forecasts.
Regarding the second part of the argument, some
worry that a policy designed to prick an emerging
bubble could have unintended negative consequences.
Opponents of bubble-popping often cite the example
of the Great Depression, claiming it was exacerbated
by the Fed’s overzealous attempts to rein in speculative
stock market excesses. A counterargument is
provided by Borio and Lowe (2002), who perform an
exhaustive historical study of financial market bubbles
in many countries.According to the authors,opponents
of bubble-popping fail to take sufficient account of
the asymmetric nature of the costs of policy errors
when faced with a suspected bubble:“If the economy
is indeed robust and the boom is sustainable, actions by the authorities to restrain the boom are unlikely
to derail it altogether. By contrast, failure to act could
have much more damaging consequences,as the imbalances
unravel” (p. 26).They also argue that emerging
bubbles can be more readily identified if central banks
look beyond asset prices to include other variables
that signal a threat to financial stability. Specifically,
they find that episodes of sustained rapid credit expansion,
booming stock or house prices, and high levels
of investment, are almost always followed by periods
of stress in the financial system.
Can bubbles be identified?Despite the many problems propagated by bubbles,some have argued that central banks should ignorebubbles.Their argument consists of two parts: (1) centralbanks cannot reliably determine if a run-up instock prices is actually a bubble until after it has burst;(2) an interest rate hike of sufficient magnitude toprick a suspected bubble would likely send the economyinto a recession, thereby sacrificing the benefitsof the boom that might otherwise continue.To identify a bubble in real time, policymakers wouldneed to judge whether a valuation metric, such as theprice-earnings (P/E) ratio for the aggregate stock market,has crossed into bubble territory. But renderingsuch a judgment may be difficult in practice,particularlyif the bubble is triggered by investor overreaction toan actual fundamental improvement in the underlyingeconomy. For example, a pickup in measured productivitygrowth during the late 1990s appeared to offerto some fundamental justification for the rise in themarket P/E ratio. Others have countered that the difficultiesin identifying bubbles do not justify ignoringthem altogether.Central banks routinely apply judgmentto a whole host of issues affecting monetary policy.One example is the size of the so-called “output gap,”defined as the difference between actual GDP andpotential GDP. The output gap is notoriously difficultto measure in real time, yet it remains an importantinput to central bank inflation forecasts.Regarding the second part of the argument, someworry that a policy designed to prick an emergingbubble could have unintended negative consequences.Opponents of bubble-popping often cite the exampleof the Great Depression, claiming it was exacerbatedby the Fed’s overzealous attempts to rein in speculativestock market excesses. A counterargument isprovided by Borio and Lowe (2002), who perform anexhaustive historical study of financial market bubblesin many countries.According to the authors,opponentsof bubble-popping fail to take sufficient account ofthe asymmetric nature of the costs of policy errorswhen faced with a suspected bubble:“If the economyis indeed robust and the boom is sustainable, actions by the authorities to restrain the boom are unlikelyto derail it altogether. By contrast, failure to act couldhave much more damaging consequences,as the imbalancesunravel” (p. 26).They also argue that emergingbubbles can be more readily identified if central bankslook beyond asset prices to include other variablesthat signal a threat to financial stability. Specifically,they find that episodes of sustained rapid credit expansion,booming stock or house prices, and high levelsof investment, are almost always followed by periodsof stress in the financial system.
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