Tải bản đầy đủ - 0trang
7 Intramarginal interventions: leaning against the wind
Figure 14.6 French franc per Deutschmark frequency, 1988–91 (daily)
considerable degree of intramarginal intervention. To put the matter another way, more
often than not what we actually see is a target zone imposed on a managed float, not on a
Suppose that instead of the intervention rule given by Equations 14.2, 14.3 and 14.4,
monetary policy is set (and publicly known to be set) so as to make the fundamental mean
3/7/14 3:45 PM
Fixed exchange rates
reverting. In other words, suppose the monetary authority tries to keep the fundamental in
the region of some central, long-run level, by changing the money stock in proportion to
how far it currently stands from that level. In practical terms, this amounts to the central
bank buying the domestic currency whenever it tends to weaken and selling whenever it is
strong – hence the expression leaning into the wind.
To make matters explicit, if the target level of the fundamental is set to zero (which
is simply a matter of appropriate scaling), then within the band the authorities follow the
Δkt = −αkt−1 + εt for . < st < | 0 < α < 1
which says that the change in k between t − 1 and t would be zero whenever the previous
period’s fundamental is spot on its target level of zero (i.e. whenever kt−1 = 0). Otherwise, it
is reduced (increased) by a proportion α of any positive (negative) deviation from its zero
target. The logic of this assumption is that although they intend to keep the fundamental as
close to its target level as possible, the authorities feel unable to react to random shocks
drastically enough to restore the situation in a single period. Instead, they restore a proportion, α. The greater (smaller) this parameter, the more rapidly (slowly) the fundamental
returns to zero.
Notice that Equation 14.8 implies:
kt = (1 − α)kt−1 + εt
At the borders of the zone, however, we assume that the rule becomes:
⎧ (1 − α)kt−1 + εt for εt < 0
whenever st = |
kt = ⎨
for εt > 0
at the upper boundary, and similarly:
⎧ (1 − α)kt−1 + εt for εt > 0
whenever st = .
kt = ⎨
for εt < 0
That is, the gradualism of the authorities gives way to more drastic measures at the limits
of the zone, where adverse shocks to the fundamental have to be completely neutralised if
the regime is to be preserved.
The result of this policy rule is illustrated in Figure 14.7. As before, the 45-degree line
shows the benchmark outcome of a free float. The line labelled MM shows what would
happen to the exchange rate in the absence of a target zone, but with the authorities
operating a policy of leaning into the wind – that is, MM shows the effect of applying the
policy rule (Equation 14.9), without Equations 14.10 and 14.11. As is intuitively obvious,
the mean-reverting fundamental generates a honeymoon effect even without any bounds
on the exchange rate’s movements. The reason is straightforward: whenever the exchange
rate is weak (above its central parity), then the fundamental is expected to fall, as the
authorities contract the money stock and this expectation itself causes the currency to
Now take account of the imposition of the target zone. It turns out to have very little
consequence, as can be seen from the curve labelled TT in Figure 14.7, which is very close
to the managed float line MM, with only very slight curvature at the boundaries. The explanation here is again straightforward. Inside the zone, the honeymoon effect operates as
3/7/14 3:45 PM
Figure 14.7 Target zone with intramarginal intervention
before. The difference between this scenario and the Krugman model of a target zone
imposed on a pure float is that the prospect of intervention at the boundaries is simply much
less important here, since the probability of it ever being required is so much smaller when
the authorities react intramarginally. It is only as the exchange rate nears one of the barriers
that the effect of the potential intervention is felt. As long as the exchange rate is well inside
the band, the contribution of the barrier intervention to the expected rate of appreciation or
depreciation is negligible, because it is so unlikely to occur, given that the authorities can be
relied on to act long before the situation gets so desperate.
The corollary of this conclusion is that the broader the fluctuation band, the less important the barriers become and the more the outcome resembles the straightforward managed
float without barriers that would result from a mean-reverting monetary policy rule.
This model, it turns out, fits the facts much better than the basic Krugman model.
Published empirical work suggests that the relationship between the exchange rate and the
fundamentals is almost linear within the target zone, with barely perceptible curvature at
the edges of the band.
The implication of this model’s relative success is plain. If almost all the benefits of a
honeymoon effect are achievable simply by leaning against the wind, then it seems hardly
worthwhile going to the trouble of imposing a target zone, with all that this entails in terms
of institutional arrangements, international commitments and investment of political and
3/7/14 3:45 PM
Fixed exchange rates
Credibility and realignment prospects
The other glaring departure from realism in the models discussed so far is in the general
area of credibility. As exchange rates approach the limits of a target zone, markets do not
typically base their expectations on a blind faith in the will and commitment of the authorities
to do what is necessary and adjust monetary policy in the required direction. On the contrary, the closer the edge of the band, the more they are inclined to question whether the
central bank is willing and able to defend the target zone, or whether it will not in the end
decide to give up the struggle. Two researchers, Bertola and Caballero (1992), showed that
the implications of incorporating these factors into the target zone model can be sufficient
to make it consistent with the facts. It is possible here to give only a broad outline of the
general framework of their model, as its detailed workings are complex and many of their
results were in any case presented only for calibrated versions of their model.
Suppose that, as in the original Krugman model, intervention takes place only at the
boundaries of the zone. Intramarginal monetary policy is dormant, and so the motion of
the fundamentals inside the zone is described by a random walk as in Equation 14.2 and
by Equations 14.3 and 14.4 at the upper and lower bounds.
Although we continue to assume that the zone is symmetrical (i.e. A14.4 still stands) in
this section, we need to introduce two new symbols. First, label the central parity ct so as to
allow for the fact that it will not normally be zero in the scenario we are currently dealing
with and will be subject to change (‘realignments’) under circumstances to be outlined
shortly. Next, define B as the maximum permitted fluctuation in the exchange rate under
the target zone – that is, the distance from the central parity to the edge of the band. Given
our assumption of symmetry, this is of course simply equal to 1/2(| − .), but the use of B will
be less cumbersome. So, with this notation, the initial target zone is from ct − B to ct + B.
Now consider what happens at t if the exchange rate reaches, say, the top edge of the
zone. Intervention is called for, but we assume that it may take one of two possible forms.
With probability p, the authorities decide not to defend the parity but instead to re-establish
the target zone around a new central rate of ct + 2B. In other words, the range of the new
zone is defined by:
. = ct + B and | = ct + 3B
so that it adjoins the current band. However, with probability 1 − p, the authorities choose not
to realign the central rate but instead act to reduce the money stock, not simply by enough to
prevent it breaking out of the zone but sufficiently to return it to the current central rate, ct.
In this model, expectations can be decomposed into two components. As before, we need
to consider the expected rate of depreciation within the band – that is, the expected rate of
change of the exchange rate, conditional on the absence of a realignment. But, in addition,
we need to take account of the expected rate of realignment at any moment, which plays
the role of an additional fundamental in the model. Indeed, its presence is the key to understanding how the model seeks to explain the facts. For example, if this model is correct, then
ignoring the existence of this factor explains the apparent failure of standard tests of UIRP
on data from target zones (notably the EMS). In analytical terms, the decomposition of
expectations turns out to be the key to analysing the model.
Consider the application of the no-arbitrage condition in the present context. For the
reasons discussed in the introduction to this chapter, we cannot allow a situation in which
3/7/14 3:45 PM
the exchange rate is expected to change by a discrete amount when intervention is imminent. This means that at the point where the exchange rate reaches the barrier, its expected
change must be zero. More formally, we require that:
p(ct + 2B) + (1 − p)ct = | = ct + B
where the left-hand side of the equation weights the future exchange rate in the event of
a devaluation to a new central rate of (ct + 2B) by its associated probability, p, and the current, unchanged central rate by (1 − p), the probability that the current zone is defended
(remember that if they decide to defend the parity, then the authorities are assumed to
intervene sufficiently to push the exchange rate all the way back down to the central rate).
In general, the conclusions of this model are complex and very much dependent on the
parameter values, especially the value of the realignment probability, p. As long as this
probability is less than 1/2, the familiar S-curve is preserved, but it gets flatter for higher
values of p. At the critical value p = 1/2, the S-curve is flattened so much that it coincides with
the 45-degree line – hence, a free float.
The most interesting cases, however, are when p > 1/2. Here, the Krugman model conclusions are reversed: the exchange rate path is everywhere steeper than the 45-degree line.
Moreover, instead of becoming flat at the margins, the exchange rate curve becomes vertical, which implies complete instability. Most importantly, the model generates a frequency
distribution that is far closer to the facts, at least in so far as it predicts that the exchange
rate will be in the central region of the band for most of the time, with few observations at
the extremes near the bounds. The reason for this conclusion is that inside the band, the
model predicts a stable, relatively placid scenario, with realignment only a distant possi
bility and hence very little currency market reaction to disturbances in the fundamentals.
By the same token, at the margins, expectations are dominated by the prospect of a devaluation or revaluation, the more so if p is high. This means that we are likely to observe the
exchange rate lingering for quite long periods in the interior of the zone, but passing rapidly
through the extreme regions, whether as a result of remedial action by the central bank or
alternatively as a consequence of the establishment of a new central parity. In either case,
the model predicts the inverted U-shaped distribution of outcomes actually seen in fixed
exchange rate regimes.
In one respect, the story of this chapter has been similar to so many others in this book. Once
again, an elegant theory appears to generate predictions that are flatly contradicted by the
data. This time, however, modifications that, in any case, bring the assumptions closer to
reality also serve to make it consistent with the facts.19
It is appropriate that the final model considered in this chapter focused attention on
the issue of credibility, because its importance in fixed exchange rate regimes of any kind is
too overwhelming to be ignored. What Bertola and Caballero (1992) ignore20 is that only
with a crawling peg regime (which is what they really model) does the target zone shift
seamlessly and painlessly from one central parity to another. Where there is no provision
for a crawling peg, fixed rate regimes change only when they are forced to do so. In reality,
most changes are brought about by crisis. The next chapter deals with the question of why
crises occur in the currency markets and how they evolve.
3/7/14 3:45 PM
Fixed exchange rates
Target zones are ranges within which fixed exchange rates are allowed to fluctuate.
When they are in force, however, and provided they are credible, they have a more
complicated effect than might be expected. In particular, they do not make the
exchange rate behave as if it were floating freely inside the zone.
Under the assumptions of Krugman’s well-known model, the exchange rate is less
volatile (i.e. reacts less to changes in the fundamentals) than under a pure float in the
interior of the zone. This extra stability is known as the honeymoon effect.
In addition, the honeymoon effect is greater the closer the exchange rate gets to its
boundaries, so that in the limit it is totally unaffected by the fundamentals when
it actually reaches its floor or ceiling. In other words, it approaches the bounds
from almost the horizontal, touching them tangentially, a property known as smooth
The holders of the domestic and foreign currency can be viewed as having undertaken
reciprocal options trades. The fact that the target zone places an upper bound on the
price of foreign currency means that domestic (foreign) currency holders have bought
(sold) a call on foreign currency with an exercise price equal to the upper bound. The
premium is paid (received) in the form of the inflated price paid for the domestic
currency, via the honeymoon effect.
The Krugman model predicts that the exchange rate will be at or near its limits most
of the time. The facts about target zone regimes suggest the exact opposite: exchange
rates tend to be in or around their central parity for most of the time.
One possible explanation of this anomaly may be that central banks are wary of waiting until the last minute to intervene, instead preferring to offset fluctuations in the
fundamentals as and when they occur, by leaning into the wind – that is, intervening
to neutralise at least partially any changes in the value of the currency long before they
drive the exchange rate to its bounds. However, once we allow for this type of policy,
the advantage of actually having a target zone vanishes almost completely, in so far
as the honeymoon effect is tiny and the exchange rate behaves more or less as it
would under a managed float.
The markets are well aware that, at any given moment, there is some non-zero probability that the authorities will decide to move the central parity and re-establish
the target zone around a new parity. With this innovation, market expectations
incorporate not simply the anticipated rate of depreciation within the target zone
but also the possibility of a devaluation or revaluation to a new level. Although the
detailed conclusions depend on the probability of a new target zone being chosen, in
general the results are potentially consistent with the facts about fixed exchange
3/7/14 3:45 PM
An excellent background text is Section 8.5 of Obstfeld and Rogoff (1996), although readers should
perhaps be warned that they cover the subject matter of this chapter and the next in reverse order
from the one chosen here – that is, they tackle crises and speculative attacks in their Section 8.4.2,
before target zones. The classic Krugman (1991) paper is a relatively easy read, although the mathematics involves a controlled diffusion process, which makes the details of the analysis difficult.
Bertola and Caballero (1992) developed the model of realignments covered in the final section of
this chapter, having first shown how and why the original Krugman model cannot explain the facts.
An intuitive exposition of all three models is to be found in Svensson (1992), which also contains a
number of references to important papers on the subject. For evidence that the honeymoon effect is
extremely small, see Iannizzotto and Taylor (1999) but see also Cerny (1999), and Flood and
The route to understanding Section 14.4 is through the vast options literature. Although I know of no
text that covers the application to target zones explicitly, there are many excellent general treatments of the binomial option pricing model, for example Chance (1998) and Hull (1998). The
original model dates back to Cox, Ross and Rubinstein (1979).
Finally, for those readers who want to master the mathematics of continuous time stochastic
processes, which features in most of the important papers in this area, the best starting point is
1 Notice that the rate of return on the transaction is infinite in any case. Not only is there no investment
required upfront to generate a positive return, but also the profit is made, in principle at least, over an
infinitely short instant of time, so that measured on any discrete time basis (e.g. annually) it is equivalent
to an infinite rate.
2 Mathematically speaking, in any case, a number or single point in a graph is infinitely small, so that there
is a sense in which it would be impossible ever to be sure that the temperature was exactly right, let alone
keep it there indefinitely. The problem is not quite as unambiguous in the case of exchange rates, since
there is in principle nothing to stop a central bank declaring its readiness to buy or sell currency at the
exact fixed rate, just as described in Chapter 5. In practice, however, a target zone is usually a more
attractive option, for a number of reasons, one of which will be explained in the next section.
3 This is the usual case in practice, though there is no reason in principle why target zones have to be
4 The normal (or Gaussian) distribution is the symmetrical bell-shaped curve that is assumed to describe,
for example, the probability distribution of people’s height about the population mean. Further details
can be found in any (randomly selected) statistics textbook.
5 In theory, the fact that any country’s reserves are finite is not a problem here, as long as there is bilateral
cooperation in maintaining the target zone, because the reserves of the strong currency country can
always be used to support the weaker currency. For example, a dollar–euro target zone could be maintained by the US Federal Reserve agreeing to provide dollars (printing them, if necessary) whenever the
euro needed support, while the European Central Bank promised to reciprocate by supplying euros to
prop up the dollar if it fell to its floor. However, as we saw during the 1992 sterling crisis in the ERM, that
presupposes the credibility of bilateral cooperation, which may also be finite.
6 See Section 11.1 for an explanation of mathematical expectations.
7 Since the example set out in this section is based on the binomial option pricing model (see the Reading
guide for references), it will look especially familiar to readers with some knowledge of the basics of
derivative pricing. However, it is intended to be intelligible to all readers, whatever their background.
3/7/14 3:45 PM
Fixed exchange rates
8 Note that we have chosen the letters u for up and d for down. The d should not be taken as indicating an
infinitely small change, as in differential calculus.
9 Option pricing buffs will notice that the values of u, d and p have been chosen to be consistent with a
zero-interest rate differential, which means we can ignore interest rates in what follows.
10 All computations in the table have been presented after rounding to two decimal places. The computations were actually generated in Excel™ to a far higher level of accuracy.
11 This assumption is an attempt to approximate two features of Krugman’s model, which are otherwise
difficult to replicate in the present setting: specifically, his use of continuous time mathematics and, in
that context, his assumption that intervention only ever occurs at ‘the very last moment’ – that is, an
infinitely small distance before the barrier is reached. So, for the purposes of the example in this section,
we replace A14.2 by the assumption that the authorities restrict themselves to bringing the exchange rate
back within the barrier. In other words, we explicitly rule out the possibility that they may make a more
drastic cut so as to bring the exchange rate to a level safely below the limit.
12 If necessary, refer back to Chapter 11, especially Equation 11.6.
13 The figure 760.28 was computed by solving Equation 14.1 for the value of k, which sets s equal to 760.
In other words, it is the value k* = (1 + b)(760) − bse.
14 Strictly speaking, of course, these high rates were not consistent with the model described here, because
they resulted from the breakdown of credibility, contravening A14.3. However, the point still stands: if
high interest rates are painful when credibility is challenged, then they could hardly be less painful when
it is unchallenged.
15 The frequency distribution of a random variable is simply the graph of the number of times each possible
value is observed in a particular sample.
16 The frequency drops to zero at −0.45% and remains at zero for all values to the right in Figure 14.6, and
so nothing is lost by ending the axis at −1.65%.
17 According to Delgado and Dumas (1991), about 85% of intervention in the EMS was intramarginal.
18 Note the implicit policy anticipation hypothesis (see Section 12.4).
19 Some readers may think it obvious that more realistic assumptions should lead to a model that better fits
the facts or indeed generates improved forecasts. In practice, this is not necessarily the case: models with
assumptions known to be unrealistic may fit the data well, while more superficially plausible models
contribute very little to explaining the data.
20 Deliberately, of course, because the model is complex enough without introducing further complications.
3/7/14 3:45 PM
Formal derivation of the model
In this chapter, the target zone model was set out in a simplified form in discrete time and
with uncertainty represented by only two alternative outcomes. This appendix presents a
more formal version of the Krugman target zone model, making use of a continuous time
stochastic model that allows for uncertainty in a far more general way.
Start with the equation determining the (log of the) exchange rate itself. In place of
Equation 14.1, we write:
s ≡ ϕ(h) = h + β
which says that at any time, t (we suppress the time subscripts where there is no danger of
confusion), the variable s is a function, ϕ(h) of the fundamental, h. Moreover, the function
is simply h plus a component that reflects the speculative demand for the currency, a positive coefficient β > 0 times the instantaneous expected rate of depreciation. The term Eds
can be thought of as the expected change over the next infinitesimal interval from t to t+, a
period of length dt.
Notice that the function ϕ(h) summarises the relationship that is the subject of the chapter.
In this appendix, we are investigating that function in order to check the conclusions we
derived more casually in the body of the chapter regarding its shape.
We assume that in the interior of the zone, the fundamental is driven by a diffusion
process – that is, a stochastic differential equation of the following form:
dh = λdt + σdω
which simply says the change in the fundamental over any tiny interval is the sum of an
expected component, λ, and a random component with a standard deviation of σ. (The final
term is a Wiener process, which is a sequence of normally distributed shocks.) It should be
clear that Equation 14.14 is the continuous time analogue of a random walk with a drift of
λ per period and a normally distributed error term.
In order to make progress, we utilise the most important theorem in stochastic calculus,
Ito’s lemma, that allows us to write the diffusion process for ds, given that the fundamental
evolves according to the rule set out in Equation 14.14. Doing so leads us to the conclusion
that the exchange rate process will be:
ds = [ϕ′(h)E(dh) + 1/2 ϕδ(h)σ 2]dt + ϕ′(h)σdω
and ϕ″(h) ≡
Notice that, as promised, this equation has the same general structure as Equation 14.14
– that is to say, it is also a diffusion process, with a drift term in the square bracket and a
residual shock involving the same Wiener process, dω. For our purposes here, what matters
is the expected change in the exchange rate, given by the term in the square brackets.
3/7/14 3:45 PM
Fixed exchange rates
Consider the situation just before a boundary of the target zone is reached. First, note
that the variance of the random fundamental must be approaching zero. Second, as already
explained, the expected change in the fundamental E(dh) will be non-zero. It follows that
we can have a zero expected change in the exchange rate only if the coefficient of E(dh) in
Equation 14.15 is also zero. So we need to have ϕ(h), the gradient of the ϕ′(h) function with
respect to the h axis, zero. In other words, the curve representing the exchange rate as a
function of the fundamental must be flat at the boundaries of the zone, which is exactly
what we have with smooth pasting.
3/7/14 3:45 PM
Crises and credibility
First-generation model* 388
Second-generation models* 395
Third-generation models 404
The 2008 crisis 410
Reading guide 415
This chapter is again concerned with fixed exchange rate regimes. This time, however, we
are concerned not with how they work but with how they fail to work and, indeed, how they
Countries rarely abandon an exchange rate peg voluntarily. More often than not, fixed
exchange rate regimes collapse when the foreign currency reserves are exhausted or at
least so depleted in such a short space of time that the government1 decides to throw in the
towel while it still has enough reserves left to allow it to intervene at some future date, for
example in the run up to a forthcoming election. The end is typically a painful, messy affair,
with the air of crisis in the currency market spreading to the rest of the country’s economic
and sometimes political life. Rather than the steady draining away of the reserves that we
might expect to see when a monetary policy is overexpansive, the end usually comes as the
result of a sudden collapse, as speculators anticipate the ultimate breakdown of the regime.
Their reaction is critical. The crisis is triggered by their rush to convert their money balances
into foreign currency, while the central bank still has reserves left to offer them. The resulting
raid devastates the reserves and leads to more or less inevitable abandonment of the
exchange rate peg.
Why do these cataclysmic events occur? Why does the end come with a bang rather than
3/7/14 3:47 PM