The Bond Yield “Conundrum”
from a Macro-Finance Perspective
Glenn D. Rudebusch, Eric T. Swanson, and Tao Wu

In 2004 and 2005, long-term interest rates remained remarkably low
despite improving economic conditions and rising short-term interest rates,
a situation that then-Federal Reserve Board Chairman Alan Greenspandubbed a “conundrum.” We document the extent and timing of this conundrum using two empirical no-arbitrage macro-finance models of the term
structure of interest rates. These models confirm that the recent behavior of
long-term yields has been unusual—that is, it cannot be explained within
the framework of the models. Therefore, we consider other macroeconomic
factors omitted from the models and find that some of these variables,
particularly declines in long-term bond volatility, may explain a portion of
the conundrum. Foreign official purchases of U.S. Treasuries appear to haveplayed little or no role.
Keywords: Term structure; Term premium; Bond pricing; Affine noarbitrage model
JEL Classification: E43, E44, G12

Glenn D. Rudebusch: Federal Reserve Bank of San Francisco (E-mail: Glenn.Rudebusch@sf.frb.org)
Eric T. Swanson: Federal Reserve Bank of San Francisco (E-mail: eric.swanson@sf.frb.org)
Tao Wu: Federal Reserve Bank of Dallas (E-mail: tao.wu@dal.frb.org)
We thank the conference participants and our discussants for helpful comments as well as Brian Sack,
John Williams, Jonathan Wright, and seminar participants at the Federal Reserve Bank of San
Francisco. Brian Sack graciously provided us with Matlab code and data. David Thipphavong
and Vuong Nguyen provided excellent research assistance. The views expressed in this paper are
those of the authors and do not necessarily reflect the views of other individuals within the Federal
Reserve System.
MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006
DO NOT REPRINT OR REPRODUCE WITHOUT PERMISSION.

83

[L]ong-term interest rates have trended lower in recent months even as the Federal
Reserve has raised the level of the target federal funds rate by 150 basis points.
This development contrasts with most experience, which suggests that, other things
being equal, increasing short-term interest rates are normally accompanied by arise in longer-term yields . . . For the moment, the broadly unanticipated behaviorof world bond markets remains a conundrum.1

I. Introduction
As a broad empirical regularity, long-term interest rates tend to move month by monthin the same direction as short-term rates, although by a lesser amount. In the UnitedStates, the simple correlation since 1984 between changes in short-term and long-term
rates implies that a 1 percentage point increase in the monthly federal funds rate has
been accompanied, on average, by about a 30-basis-point increase in the 10-year rate.
However, in 2004 and 2005, long-term interest rates followed a very different pattern.
For example, while the federal funds rate steadily rose from 1 percent in June 2004
to 4.2 percent in December 2005, the rate on 10-year U.S. Treasury notes fell from
4.7 percent to 4.5 percent over that same period.2 This directional divergence between
U.S. short and long rates appears even more unusual given other pressures at the time,such as a robust economic expansion, rising energy prices, and a deteriorating federal
fiscal situation, all of which have tended to boost long-term interest rates in the past.
In this paper, we investigate the seemingly odd behavior of long-term interest rates
over this recent episode, a development which then-Fed Chairman Alan Greenspanlabeled a “conundrum.”3
Determining whether recent long-term interest rate movements truly represent
a conundrum—as opposed to simply an extension of secular declines in long-term
interest rates that have been taking place over the past 20 years—requires a rigorous
theoretical framework which can take into account the important factors that affectlong-term rates. Our choice of a theoretical framework for this examination is guided
by much recent research that suggests a joint macro-finance modeling strategy provides
the most comprehensive explanation of movements in long-term rates.4 From a macroeconomic perspective, the short-term interest rate is a policy instrument under the
direct control of the central bank, which adjusts that rate to achieve its economic
stabilization goals. Therefore, financial market participants’ understanding of central
bank behavior will be an important element in the formation of their expectationsof future short-term interest rates, which, in turn, will be a key component for the
1. Testimony of Federal Reserve Board Chairman Alan Greenspan to the U.S. Senate, February 16, 2005.
2. In addition, given the upward shifts in money market futures rates during 2005, the extent of the policy tightening
appears to have been unanticipated.
3. Although Greenspan referred to “world” bond markets, our focus is exclusively on U.S. markets. For a discussionof recent bond rates in Japan from a macro-finance perspective, see Oda and Ueda (2005).
4. The connection between the macroeconomic and finance views of the term structure has been a very fertile
area for recent research, which includes, among many others, Ang and Piazzesi (2003), Wu (2002), Dewachterand Lyrio (2006), Evans and Marshall (2001), Diebold, Rudebusch, and Aruoba (2006), Hördahl, Tristani, andVestin (2006), Rudebusch and Wu (2004, 2006), Kozicki and Tinsley (2001, 2005), and Diebold, Piazzesi, andRudebusch (2005).

84 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

pricing of longer-term bonds. To illustrate this point, it is useful to contrast the recent
behavior of U.S. interest rates with their behavior during the previous episode ofextended monetary tightening, which occurred a decade earlier. Specifically, fromJanuary 1994 to February 1995, as the Fed raised the short-term federal funds rate by
3 percentage points, the 10-year rate also rose by 1.7 percentage points. This reaction
was somewhat greater than the average response, and a common interpretation of this
episode is that bond investors were especially worried that inflation pressures might
not be counteracted by the Fed in a timely fashion. In contrast, there is a widespreadview that long-term inflation expectations remained well contained in 2004 and 2005,
which helped hold down long bond rates. Understanding the differential behavior of
long-term interest rates during these two episodes likely requires an appreciation of themacroeconomic underpinnings of bond rate dynamics.
A finance perspective, which stresses the importance of changing investor perceptions of risk for bond pricing, is also likely to be a crucial element in assessing whether
there is any bond rate conundrum. Indeed, many have suggested that a reduction in
the risk premium is responsible for recent low levels of bond rates. Such a reduction
may be attributable to changes in the amount of risk or to changes in the pricingof that risk, and numerous factors have been suggested that could have induced
such changes. For example, a new appreciation of lower macroeconomic volatility or
reduced monetary policy uncertainty could alter assessments of the amount of interest
rate risk faced by investors. Alternatively, risk aversion may have been reduced as
new global investors entered the market. Indeed, this is one interpretation of the
explanation suggested by Bernanke (2005), who argued that “over the past decadea combination of diverse forces has created a significant increase in the global supplyof saving—a global saving glut—which helps to explain both the increase in the U.S.
current account deficit and the relatively low level of long-term real interest rates in
the world today.” In any case, it is likely important to allow for time variation in risk
premiums in understanding the recent behavior of the bond rate.
To conduct our analysis of the conundrum, we use two macro-finance models of
the term structure from the literature. The first is a vector autoregression (VAR)-basedmodel developed by Bernanke, Reinhart, and Sack (2004), denoted BRS (2004), andthe second is a New-Keynesian-based model developed by Rudebusch and Wu (2004),
denoted RW (2004). The BRS and RW models share several basic features. First, they
are both factor models, so only a small number of sources of variation underlie thepricing of the entire term structure of interest rates. Second, both models impose thestandard no-arbitrage restriction from finance (e.g., Duffie and Kan [1996]), which
ensures that, after accounting for risk, the dynamic evolution of yields over time andacross states of nature is consistent with the cross-sectional shape of the yield curve at
any one point in time. Finally, both models have important bidirectional linkages
between interest rates and macroeconomic variables.
Despite their broad similarities, the BRS and RW models also have technicalspecifications that differ in important ways. Because these differences may lead todifferent results, we use both models in our analysis to provide us with additional
perspective and robustness checks on our results. Specifically, we extend bothmodels forward from their original samples to the 2004–05 “conundrum” period to85

determine whether the recent low level of long-term bond yields was unusual fromthe viewpoint of the models. Our analysis indicates that the level of long-term bondyields in 2004 and 2005 is, in fact, substantially lower than can be explained byeither of these models—in other words, that this period does constitute a conundrum
from a macro-finance perspective.
Having documented the existence of a conundrum within a rigorous econometricframework, we then turn to possible explanations—factors that necessarily lie outsideof the two macro-finance models we have considered. We examine several popular
explanations for the conundrum that have been promoted by financial analysts andfind that one of them—lower volatility of long-term Treasury yields—seems to havesubstantial explanatory power. Interestingly, we find no support for the view that
foreign official purchases of U.S. Treasuries have contributed to the low level oflong-term yields, even though this factor is regarded by many financial market
participants to have been the single most important factor holding down long-term
U.S. Treasury yields.
Although there are numerous papers specifying finance and macro-finance modelsof the term structure, our paper fills a gap in the literature by applying these models toanalyze the bond yield conundrum. The most closely related paper to the present one
is Kim and Wright (2005), who use a pure finance three-factor model of the term
structure to analyze the recent low level of long-term rates and find that a declining
term premium is the key factor underlying those low rates. By contrast, we use two
macro-finance models of the term structure and find that, within the framework ofthese models, there is in fact a conundrum—that the model-implied term premiums
from our two models are unable to explain the low level of long-term yields observedin 2004–05, despite the fact that the models provide an otherwise excellent fit to
the data over the previous 20 years.
The remainder of the paper proceeds as follows. In Section II, we describe the
BRS and RW macro-finance models of the term structure. In Section III, we discuss
the estimation of these models and use them to show that there was a bond yield
conundrum in the 2004–05 period. In Section IV, we investigate what other factors
outside of the two models could potentially help to explain the bond yield conundrum
episode. Section V concludes.

II. Macro-Finance Models of the Term Structure
To investigate whether and to what extent the low level of U.S. long-term interest
rates can be explained by macroeconomic conditions, we will use two macro-financemodels of the term structure, broadly following Ang and Piazzesi (2003), Piazzesi(2005), and many others. For analyzing the behavior of the yield curve, these macrofinance models offer a number of advantages over both pure finance models and puremacroeconomic models.
First, in contrast to standard finance models of the term structure (e.g., Dai
and Singleton [2000]), which relate the yield curve to current and past interest rates,
macro-finance models recognize that interest rates and macroeconomic variables

86 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

evolve jointly over time, with feedback running from interest rates to macroeconomic
variables and also from macroeconomic variables back to interest rates. The latterchannel in particular is crucial for the behavior of short-term interest rates, which
are determined in many countries by the central bank as a function of the state ofmacroeconomic variables such as the output gap and inflation.5
A second advantage of macro-finance models is that they allow the behavior of risk
premiums to depend explicitly on macroeconomic conditions. Standard consumptionbased models of asset returns imply that risk premiums are determined by thecovariance of an asset’s return with the marginal utility of consumption (e.g., Cochrane[2001]). Moreover, empirical studies of excess returns in bond markets and shorterduration interest rate derivatives (Cochrane and Piazzesi [2005] and Piazzesi andSwanson [2004]) find a strong countercyclical relationship between economic activity
and excess returns on these securities, particularly during recessions. Macro-financemodels of the term structure explicitly allow for a relationship between risk premiums
and macroeconomic conditions.
A third advantage of macro-finance models is that, in contrast to standard macroeconomic models, a substantial component of observed bond yields is allowed toreflect term or risk premiums that may vary substantially over time. Indeed, as in afinance model, macro-finance models allow term premiums to evolve according to
the estimated dynamics of the model in a way that is consistent with an absence ofarbitrage opportunities in financial markets. Thus, while macro-finance models of
the term structure retain the appealing macroeconometric features of a dynamic
stochastic general equilibrium (DSGE) model or a VAR, they do not impose theexpectations hypothesis and can allow for a rich behavior of term premiums.
To explore the recent behavior of long-term bond rates, we use two macro-financemodels from the literature, the VAR-based model developed in BRS (2004) and themore structural New Keynesian-based model developed in RW (2004). We discusseach in turn.
A. The BRS Model
The BRS model uses five observable macroeconomic variables, specified below, as
factors with which to fit the yield curve. The dynamics of the five factors aregoverned by a VAR with four lags, which can be stacked into the 20-element vector
X t and described by a VAR(1) model:
X t =  + X t −1 + t ,(1)

where the stochastic shocks t are i.i.d. over time and have a standard normal
distribution. Consistent with the companion form specification in (1), only theuppermost blocks of  and  are nontrivial. For simplicity, the prices of risk in the
5. While a reduced-form relationship between past interest rates and the future behavior of short-term interest rates
exists, the Lucas critique suggests that it may be unstable in the face of changing monetary policy. A more structural
macro-finance approach that models monetary policy is arguably less subject to this criticism. Kim and Wright(2005), who examine the recent conundrum with a standard finance model, may alleviate this problem by using afairly short estimation sample (since 1990).

87

model, specified below, are assumed to load only on the five current values of themacroeconomic variables (i.e., the top five elements of X t ).
Following Duffie and Kan (1996), Dai and Singleton (2000), and Ang and Piazzesi(2003), among others, we assume that the stochastic discount factor with which bondsin the model are priced is conditionally log-normal with functional form:6
mt +1 = exp(−i t − ′t t /2 − ′t t +1),

(2)

where t is a 20-dimensional vector of market prices of risk associated with theinnovations to the VAR, t . Of course, because only the first five elements of t
are nonzero, we assume that only the first five elements of t are nonzero, which isessentially a normalization. Following the papers cited above, the market prices ofrisk t are assumed to be affine functions of the state variables of the economy:

t = 0 + 1X t ,(3)where 0 is a 20-dimensional vector of constants and 1 is a 20-by-20 matrix of risk
price factor loadings on the state variables X t . As mentioned above, we assume forsimplicity that the prices of risk depend only on the current values of the variables in
X t , thus only the upper-left five-by-five block of 1 is nonzero.
Bonds in the model are priced according to the standard relationship with thestochastic pricing kernel:
Pt n = Etmt +1Pt +1n −1,(4)where Pt n denotes the price of an n-period zero-coupon bond at date t. Let ytn denotethe corresponding continuously compounded yield on the same zero-coupon bond.
Then it follows from equations (1)–(4) that1
ytn = ––(an + bn′X t ),n(5)where a 0 = 0, b0 = 0, and an (a scalar) and bn (a five-by-one vector) are computed
recursively according toan +1 = 0 + an +bn′( − 0) − bn′ ′bn ,bn +1 = 1 + ( − 1)′bn ,(6)

and where 0 (a scalar) and 1 (a five-by-one vector) describe the one-period interest
rate as a function of the state of the economy: yt1 ≡ i t = 0 + 1′X t .
6. It is a well-known and important result from finance that the assumption of no arbitrage (or even more weakly, thelaw of one price) implies that a stochastic discount factor must exist, even in an incomplete market—see, forexample, Cochrane (2001) for discussion. The papers cited here place the additional functional form restriction on
the pricing kernel for tractability.

88 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

B. The RW Model
The RW model shares many features with the BRS model, since both models pricebonds using an affine no-arbitrage framework. Thus, for expositional simplicity andto conserve notation, when concepts in the two models are the same, we will take thecorresponding variable name from the BRS model exposition above and recycle it in
the description of the RW model below (even though the number of factors, andhence the number of dimensions, in the two models are different). Note that, despite
the similarity in the bond-pricing framework, the dynamics of the underlying macroeconomic factors in the RW model differ in important ways from BRS, with RW
using a more structural New Keynesian macroeconomic framework to model these
dynamic relationships as opposed to a VAR.
Bond pricing in the RW model is governed by the same affine no-arbitrage framework in (2)–(6) described above. However, in contrast to the BRS model, the RW
model defines the one-period interest rate to be a sum of two latent factors, L t and St :i t = 0 + L t + St ,(7)

where L t can be thought of as the “level” of the yield curve and St as (the negativeof ) the yield curve “slope,” as discussed in RW (2004).7 Intuitively, the Fed sets the
one-period nominal interest rate in the model as the sum of a constant steady-state
real interest rate (0), a time-varying medium- to long-term inflation rate (L t ), and acyclically responsive component St that is given by
St = S St −1 + (1 − S )[g y yt + g ( t −L t )] + uS,t ,(8)

uS,t = uuS,t −1 + S,t ,(9)where yt is the output gap and t the inflation rate. Equation (8) is essentially a
Taylor (1993) rule for the short-term rate i t with both inertia (through the lag St −1)
and serially correlated errors (as described by Rudebusch [2002b]).
The dynamics of L t are given by
L t = L L t −1 + (1 − L ) t + L,t ,(10)which says that the medium-term inflation goal is persistent but may depend also on
the recent behavior of short-term inflation, as suggested by the evidence in Gürkaynak,
Sack, and Swanson (2005).
The latent factors Lt and St are jointly determined with the macroeconomic variables
yt and t by a New Keynesian-type model (adjusted to apply to monthly data):t=  L t + (1 –  )[

1

t –1

+

2

t –2

]+

y +  ,t ,y t –1(11)

7. RW (2004) present charts comparing their estimated values for L t and St to traditional principal components decompositions of the yield curve into “level” and “slope” factors, and show that the two sets of series line up very closely,
justifying the use of the terminology.

89

yt = y Et yt +1 + (1 – y )[ y1 yt –1 +y ]–y 2 t –2r

(it –1 – Lt –1) +  y,t .

(12)That is, inflation responds to the private sector’s expectation of the medium-term
inflation goal (L t ), two lags of inflation, and the output gap; the output gap, in turn,responds to the expected future output gap, two lags of the output gap, and a realinterest rate. Equations (11) and (12) are “hybrid” New Keynesian equations in thesense that the expectational coefficients  and y are allowed to be less than unity
when the model is fitted to the data.

III. Is There a Bond Yield “Conundrum”?
To investigate whether the recent low level of U.S. long-term bond yields is a conundrum from a macro-finance perspective, we use the BRS and RW models to fit the
recent macroeconomic and interest rate data and examine the two model predictions
for long-term bond yields.
A. Model Estimates
1. BRS model estimates
Our original inclination was to use the “off-the-shelf ” BRS model parameter estimatesfor our analysis. However, after examining the data and model estimation code (kindly
supplied by Brian Sack) and fine-tuning the nonlinear optimization procedure, wewere able to obtain a better fit to the data than reported in BRS (2004), with rootmean-squared errors (RMSEs) that were about half as large even over the identicalsample. Thus, we do not use the parameter values as estimated by BRS, since we havefound that the model can fit the data better than originally reported. Moreover,because we are reestimating the parameters of the model, we take the opportunity toextend the BRS estimation period through the end of 2005, which has the added
advantage of giving the model the best possible chance of fitting the recent low“conundrum” level of bond yields by fitting them in rather than out of the sample.
In every other respect, however, we have followed BRS (2004) in their analysis.
We estimate the BRS model on monthly macroeconomic and bond yield data overthe period from January 1984 to December 2005.8 Ideally, one would like to performthe estimation over the whole postwar period, but the preponderance of empiricalevidence suggests that the relationships between interest rates and macroeconomic
variables have not been stable over the past 40 years. In particular, many authors(e.g., Fuhrer [1996], Bernanke and Mihov [1998], and Clarida, Galí, and Gertler[2000]) have found that there was an important structural break in the conductof monetary policy around 1980, when Paul Volcker became Chairman of the Fed.9
As a result, macro-finance models estimated over the entire postwar period would
likely be subject to the Lucas critique, so we avoid this issue by beginning ourempirical estimation in January 1984, after the Volcker disinflation, after the structural
8. Because the VAR has four lags, this requires using macroeconomic data back to September 1983.
9. Evidence of a structural break in the conduct of monetary policy is also indirectly supported by the greater stabilityof inflation expectations that seem to have held in the United States in the 1990s and 2000s relative to the 1970s.

90 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

breaks in monetary policy found by the authors above, and after the structural breakin U.S. GDP volatility found by McConnell and Perez-Quiros (2000) and others.
Following BRS (2004), we use five macroeconomic variables as state variables of themodel, which are the affine factors for pricing bonds: the federal funds rate, thedeviation of employment from trend (where trend is measured using an HP filter),the year-on-year percentage change in the core personal consumption expenditures(PCE) deflator, the Blue Chip survey of inflation expectations for the upcoming year,
and the rate on the Eurodollar futures contract with four quarters to expiration.
The last variable in particular is included to capture aspects of the stance of monetary
policy that may not be adequately represented by the current and past federal funds
rate, employment gap, and inflation alone, such as financial market expectations
for the future path of policy over the next 12 months. For example, the “deflation
scare” episode in the United States in the summer of 2003 led the Fed to include
greater forward-looking language in its monetary policy announcements, such as the
now-famous “considerable period” phrase,10 which helped to shape financial market
expectations for the future path of policy in a way that was perhaps different fromwhat the past behavior of output, inflation, and interest rates would have suggested.
The inclusion of the four-quarter-ahead Eurodollar futures rate thus helps to capturechanges in the expected future path of policy that may not be adequately accounted forby the lags of the VAR. Similarly, the inclusion of the Blue Chip inflation expectations
survey helps to capture changes in the expected future course of inflation that may not
be adequately captured by the VAR. Of course, the inclusion of these forward-looking
expectational variables in the VAR creates a tension or internal inconsistency between
the forecast data series that are included as variables and the forecasts that would
be implied by the model itself based on the VAR. Although this technical issue can beaddressed by estimating the model in such a way as to ensure consistency between
the model and survey forecasts to the greatest extent possible (see Swanson [2006]),
doing so would require moving away from standard VAR estimation procedures, which
is beyond the scope of the present paper. Thus, we follow BRS (2004) and simply
estimate a VAR on all five of the macroeconomic time series above, and leave the issueof consistency between model-based forecasts and forecast data to subsequent work.
Although the BRS model (and also the RW model) is linear in the state variables,
the model’s predictions for bond yields are highly nonlinear in the parameters (asare the impulse response functions in a VAR, for example).11 Estimation of the bondpricing implications of the model is thus highly nonlinear and can be tricky inpractice, with a great many local minima. (Indeed, as noted above, by experimenting
and exploring the parameter space, we were able to obtain a better fit to the data
than reported by BRS [2004].) To reduce the number of model parameters that mustbe estimated nonlinearly, we follow Ang, Piazzesi, and Wei (2006) and BRS (2004)
10. On August 12, 2003, the Federal Open Market Committee (FOMC) released a statement that, although it wasmaintaining its federal funds rate target at the level of 1 percent, it judged “that, on balance, the risk of inflation
becoming undesirably low is likely to be the predominant concern for the foreseeable future. In these circumstances,
the Committee believes that policy accommodation can be maintained for a considerable period.”
11. Indeed, to compute the model-implied 10-year yield, we must project the monthly VAR and prices ofrisk forward 120 months. Thus, when fitting the model to the data, we are essentially trying to minimize a 120thdegree polynomial.

91

and estimate the model in two stages: first, we estimate the VAR on the five macroeconomic time series above (with four lags) over the 1984–2005 period by ordinary
least squares; and second, we take the VAR coefficients as given and estimate thestochastic pricing kernel factor loadings (which, between 0 and 1, comprise 30
parameters) using nonlinear least squares to fit our bond yield data over the same
period, 1984–2005. As in BRS (2004), we estimate the model to match the six-monthand one-, two-, three-, four-, five-, seven-, and 10-year yields as closely as possible, withequal weight on fitting each of those maturities. These yield curve data are continuously compounded zero-coupon yields estimated by the Fed from off-the-run Treasury
coupon securities (for details, see Gürkaynak, Sack, and Wright [2006]); following
BRS (2004), we use the monthly average values of these yields.12
Our results from estimating the BRS model are reported in Tables 1, 2, and 3.
The VAR parameter estimates in Table 1 show that each variable in the VAR has a
Table 1 BRS VAR Parameter Estimates
VAR coefficient estimates ()
yt
yt –1
1.078
yt –2
0.171
yt –3
–0.102yt – 4–0.1800.022
t –1–0.036t –2–0.071
t –3
0.073
t –4
ˆtBC–1–0.094
ˆtBC
0.273
–2
ˆtBC
–0.239–3
ˆtBC– 40.063
i t –10.010
i t –20.070
i t –3–0.092
it –4
0.018
iˆted
–1
iˆted
–2
iˆted
–3
iˆted
–4
Constant

0.029–0.005–0.020–0.003–0.006ˆtBCiˆt ed0.266
0.254–0.283–0.2780.311–0.033–0.234–0.068–0.1040.208
0.222–0.1900.105–0.3100.367–0.073

0.078
0.138
0.062
–0.2490.883–0.0670.187–0.0160.083–0.0860.124–0.114–0.1070.120–0.1060.093–0.0800.228–0.055–0.0940.059–0.012
0.001
0.026
1.027
–0.118–0.0210.005
0.089–0.1300.098–0.054

it
0.027
0.123
–0.038–0.1030.170–0.141–0.000–0.0120.078–0.1740.341–0.2631.311–0.4130.177–0.115–0.0070.007–0.043
0.036
0.039

0.064
–0.071
0.017–0.0040.046

0.162–0.065–0.029–0.0340.006

1.097–0.2640.139–0.1190.087

000.08080.01680.02830000.15390.115000000.3826tCholesky-factored residual variance () (Var = ′)0.086700.00340.1618–0.0001
0.01110.0047–0.01050.08000.0309

12. The finance and macro-finance literatures typically use end-of-month yield data rather than month-average
yields. However, the model is linear in the state variables, so it is completely consistent to view the model’s

92 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

Table 2 BRS Model Risk Factor Loadings





0

1

–1.998–2.349–20.822
3.366
2.525

0.473
0.712
13.846–1.814–1.0640.290
0.666–0.3230.019–0.465–1.2926.036
1.028–0.287–1.1742.517–2.0224.941–0.939–0.0300.209
0.256
12.002–1.720–1.186

Table 3 BRS Model Prediction Errors
Treasury maturity
Six months

Model prediction error (RMSE, basis points)
29.52

One year
Two years
Three years
Four years
Five years
Seven years
10 years
Average

33.35
26.98
26.58
27.78
30.03
34.45
39.30
31.00

coefficient near unity on its own first lag, with smaller coefficients on longer lags ofitself and on lags of other variables. The sum of the coefficients on each variable’s own
lags is near unity. For the federal funds rate, the coefficients suggest a hump-shaped
impulse response and autocorrelation functions, but the other variables exhibit
impulse response and autocorrelation functions that are closer to a geometric decay.
The BRS model’s estimated risk factor loadings are reported in Table 2. Althoughthese estimated loadings show the greatest coefficients on the Blue Chip inflation
expectations variable, that variable also has a relatively lower variance, as can be seenin Table 1, so the net effect on bond prices is not as great as the factor loadings would
suggest by themselves.13
Table 3 reports the quality of the model’s fit, in terms of RMSEs for the eight
points on the yield curve that the model was estimated against. Except at the shortest
horizons, these RMSEs are less than half as large as those reported by BRS (2004) for
their sample period. This difference is not simply due to the sample period: evenwhen we restrict our sample to the one used by BRS (2004), we are able to obtain
RMSEs less than half as large by experimenting and exploring the parameter space.
2. RW model estimates
For the RW model, we take the original parameter values as estimated by RW (2004)
from January 1988 to December 2000 using monthly data. In that estimation, theequations as holding for the month-average data as well as the daily data and the end-of-month data. Moreover,
our macro data are typically monthly averages, so using month-average yields is arguably more consistent with themacro data.
13. It would be desirable to perform a variance decomposition of the 10-year Treasury term premium on the five
stochastic shocks  . However, the term premium at these longer horizons is highly nonlinear in the parameters ofthe model, so that a direct computation of the variance decomposition is impossible and the delta method is likely
to be a very poor approximation.

93

inflation rate was measured by the year-on-year change in the overall PCE deflator,
the output gap was measured by capacity utilization, and interest rates were end-ofmonth data on five U.S. Treasury zero-coupon yields that have maturities of one,
three, 12, 36, and 60 months (yields are unsmoothed Fama-Bliss data expressed at anannual rate in percent).
Since there are two underlying latent factors but five observable yields, RW (2004)
follow the usual strategy and assume that the three-, 12-, and 36-month yields aremeasured with i.i.d. error, as in Ang and Piazzesi (2003). (Note that this implies that,for a given set of parameter values, the latent factors L t and St can be backed out perfectly from the observed one-month and 60-month bond yields.) The estimated sizeof the measurement error that is required to fit the other yields is a common metricfor the quality of the model’s fit. Also note that the 10-year rate is not used in theestimation, so examining the recent episode of low bond rates is an out-of-sample
exercise in terms of both the estimation sample and bond maturity.
In contrast to the BRS model, all of the parameters of the RW model are
estimated in a single step by maximum likelihood. Table 4 reports the RW model
parameter estimates, from RW (2004). First, consider the dynamics of the factors.
The factor L t is very persistent, with a L estimate of 0.989, which implies a small
but significant weight on actual inflation. The dynamics of St are also very persistent,
but this persistence does not come from partial adjustment since the S estimate isa minuscule 0.026. Instead, St responds with only a very short lag to output andinflation shocks.
Table 4 RW Model Parameter Estimates
Factor dynamics
LS
u
Inflation dynamics
yOutput dynamics
yr

0.989 (0.0068)
0.026 (0.0111)
0.975 (0.0062)

ggy

0.074 (0.0113)
0.014 (0.0074)

1.253 (0.0066)
0.200 (0.0066)

1
2

0.009 (0.0066)
0.089 (0.0067)

y1
y2

1.154 (0.0525)–0.155 (0.0066)
0.918 (0.0604)
0.078 (0.0066)

Risk price (1)
L,t
S,t

Lt
–0.0045 (0.0068)–0.0223 (0.0064)

St
0.0168 (0.0068)
0.0083 (0.0067)

Standard deviations
0.342 (0.0089)
0.559 (0.0313)
Standard deviations of measurement error
Three-month
0.288 (0.0162)
12-month0.334 (0.0194)
36-month0.127 (0.0094)
LSy

Note: Standard errors of the estimates are in parentheses.

94 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

0.238 (0.0110)
0.603 (0.0128)

The Bond Yield “Conundrum” from a Macro-Finance Perspective

The persistence in St reflects the fact that the Fed adjusts the short rate promptly
to various determinants—output, inflation, and other influences in the residual ut —that are themselves quite persistent (e.g., u = 0.975).14 The monetary policy interpretation of the slope factor is supported by the values of the estimated inflation andoutput response coefficients, g and g y , which are 1.253 and 0.200, respectively. Theseestimates are similar to the usual single-equation estimates of the Taylor rule during
this sample period (e.g., Rudebusch [2002b]).
The estimated parameters describing the inflation dynamics also appear reasonable.15 In particular, the estimated weight on explicit forward-looking expectations indetermining inflation,  , is 0.074. Since this estimate is based on monthly data, withtime aggregation, it implies a weight of about 0.21 on the interim inflation objectiveat a quarterly frequency. This estimate appears consistent with many earlier estimatesobtained using a variety of different methods and specifications. For example, using
survey data on expectations, Rudebusch (2002a) obtains a broadly comparable estimate of 0.29, which is in the middle of the range of estimates in the literature.
However, by using the yield curve to extract inflation expectations, our estimates bringnew information to bear on this important macroeconomic question.
The estimated parameters describing the output dynamics also fall within reasonable
ranges.16 Specifically, the estimated value of y = 0.009 implies a negligible weight at aquarterly frequency on forward-looking output expectations in the determination of
output behavior. This is very much in accord with the maximum likelihood estimation
results reported by Fuhrer and Rudebusch (2004).
The risk price matrix ( 1) appears significant, and the model fits the three-month,12-month, and 36-month yields with measurement error standard deviations comparable to others in the literature. For instance, the standard deviation of the three-monthyield is 29 basis points (annualized), compared with 30 basis points in the VAR-based“macro lag model” in Ang and Piazzesi (2003). The standard deviation of the 36-monthyield is 13 basis points, the same as in Ang and Piazzesi (2003).
To consider the bond yield “conundrum” in 2004–05, we must extend the RW
model forward to include this period. We do so as follows. As mentioned above, wetake the parameter values as estimated from RW (2004). We then extend the output
gap and inflation data forward to the end of 2005. For consistency with our BRS
model analysis, we use the continuously compounded zero-coupon yield curve datafrom the Fed for the period January 1988 to December 2005 (although we use themonth-end values of these data rather than the month average). From the one-monthand five-year zero-coupon yields, we can back out the RW model’s level and slope
factors, L t and St , over the extended sample 1988–2005, as noted above. (Over theperiod 1988–2000, these agree closely with the original values for L t and St in RW
14. Thus, our estimate of S decisively dismisses the interest rate smoothing or monetary policy inertia interpretationof the persistence in the short rate.
15. After taking into account time aggregation and the higher cyclical variability of capacity utilization comparedwith the output gap, the elasticity of inflation with respect to output ( y = 0.014) appears about half the size of
estimates that use the entire postwar sample of quarterly data, for example, Rudebusch (2002a). The estimate
does appear more in line with estimates obtained in recent shorter samples (Rudebusch [2001]).
16. The interest rate sensitivity of output ( r = 0.089), after taking into account the time aggregation and the use of
capacity utilization rather than the output gap, appears broadly in line with estimates that use the entire postwar
sample of quarterly data.

95

(2004), but they are not identical.) Having obtained L t and St over the full sample, andgiven the parameters of the model as estimated by RW (2004), we can then computethe model’s implications for bond yields of all maturities from 1988–2005.
B. Macro-Finance Model Analysis of the 10-Year Treasury Yield
Before analyzing the recent episode of bond yields within the framework of the macrofinance models above, it may be useful to summarize the salient differences between
the two models. Our estimates of the BRS model use monthly average interest rate
data and include the 2004–05 period in the estimation sample. The RW model
estimates use end-of-month interest rate data and are based on a sample that ends in2000. (The in-sample/out-of-sample distinction is reinforced because the BRS model
includes the 10-year yield in the estimation, while the RW model does not.) The
BRS model is a completely autoregressive, nonstructural specification (except for the
no-arbitrage assumption), while the RW model is more tightly parameterized with identifiable, explicitly forward-looking aggregate demand, aggregate supply, and policy
rule equations. As an indication of the differences in parameterization, the BRS modelhas 100 parameters describing the factor dynamics and 25 risk-pricing parameters,
excluding constants, while the RW model has 13 and four, respectively. Again, we view
these differences between the two models as useful to the extent they can illuminate therobustness of our results.
To start our analysis, we plot the 10-year zero-coupon U.S. Treasury yield together
with the decomposition of that yield into its constituent components, as implied by
the BRS and RW macro-finance models, in Figures 1 and 2, respectively. We focus on
the 10-year yield in particular because that yield was the benchmark for long-term
interest rates in the United States over the period of interest, and because discussionsof the long-term bond yield “conundrum” by policymakers and in the popular press
have often focused on the 10-year U.S. Treasury in particular.
In Figure 1 [1], we plot the zero-coupon U.S. Treasury yield from 1984 through 2005
together with the BRS model decomposition of that yield. The BRS model-implied
risk-neutral rate (the gray solid line) is the model’s estimated yield on a riskless 10-yearzero-coupon bond at each date t in a hypothetical world in which the prices of risk t
are always equal to zero and the state variables of the economy are governed by the VAR
in Table 1. The BRS model-implied 10-year Treasury yield (the gray dashed line in
Figure 1) is the model’s estimated value of the same 10-year zero-coupon bond when
the prices of risk t are no longer zero, but are instead the estimated affine function of
the macroeconomic variables given in Table 2. The BRS model-implied term premium(the black dotted line) is the difference between the gray dashed line and the gray solidline, and can be interpreted as the model’s estimate of the risk or term premium on the
10-year zero-coupon bond at each date t. Finally, the BRS model does not match thedata perfectly, so the model’s residuals—the difference between the model predictions(the gray dashed line) and the data (the black solid line)—are graphed in Figure 1 [2].
As can be seen in Figure 1 [1], the close fit of the BRS model to the data is striking.
Not only does the model capture the general downward trend in the 10-year yieldover this period, but even the high-frequency swings in this yield in the late 1980s andmid-1990s are matched extremely well. The fit is even more remarkable in light of

96 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

Figure 1 Ten-Year Treasury Yield and BRS Model Implied Values[1] Ten-Year Treasury Yield and BRS Model Decomposition
14

Percent
10-year Treasury yield
Model-implied risk-neutral 10-year yield
Model-implied 10-year term premium
Model-implied 10-year Treasury yield

12
10
8
6
4
2

0
1984 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 2000 01 02 03 04 05[2] BRS Model Residuals
150

Basis points

100

50

0

–50

–100
1984 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 2000 01 02 03 04 05

the fact that the model was not optimized to fit the 10-year yield, but rather placed
equal weight on eight maturities all along the term structure.
According to the model, both the risk-neutral 10-year yield and the term premium
have fallen over our sample, with the fall in the term premium being somewhat more

97

Figure 2 Ten-Year Treasury Yield and RW Model Implied Values[1] Ten-Year Treasury Yield and RW Model Decomposition
10

Percent
10-year Treasury yield
Model-implied risk-neutral 10-year yield
Model-implied 10-year term premium
Model-implied 10-year Treasury yield

9
8
7
6
5
4
3
2
1
0
1988 89

90

91

92

93

94

95

96

97

98

99 2000 01

02

03

04

05

93

94

95

96

97

98

99 2000 01

02

03

04

05

[2] RW Model Residuals
60

Basis points

40

20

0

–20

–40

–60
1988 89

90

91

92

important. According to the model, the risk-neutral yield has fallen about 250 basis
points over this period while the term premium has fallen about 350 basis points, andthe term premium has fallen from about one-half of the total 10-year U.S. Treasury
yield in the 1980s to about one-third of that yield more recently.

98 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

However, despite the model’s excellent fit to the data overall, the recent periodof low 10-year yields is one episode that the model notably fails to fit. The model’s
residuals from mid-2004 through the end of 2005 have typically been around 50 basis
points, and have been even greater over almost all of 2005. Although the model has
failed to fit the data on a few other occasions as well—most notably at the end of 1984,
in 1985, and from 1997 to 1999—these previous episodes were typically either muchbriefer (late 1984) or milder in size (1997 to 1999) than the latest episode. Indeed, themost recent episode’s residuals of 50 to 75 basis points are even more remarkable inlight of the very low level of long-term yields—thus, while the model’s residuals
amounted to about one-10th the level of the 10-year Treasury yield in 1984 and 1985,
and about one-15th in 1997–98, the model’s residuals have been about one-sixth thelevel of the 10-year Treasury yield in the most recent period.
In Figure 2, we present the analogous pair of graphs for the 10-year bond yielddecomposition implied by the RW model. Again, the fit of the model to the data
is excellent, and this is all the more remarkable given that the RW model was not
optimized to fit the 10-year yield at all—indeed, RW (2004) chose the five-year yieldas the longest maturity in the estimation.
A striking observation from Figures 1 and 2 is that, although both the RW andBRS macro-finance models produce very good fits to the 10-year Treasury yield,
the two models’ implied decomposition of that yield into expected short rate andterm premium components is very different. In the RW model, the term premium isrelatively constant over the 1988–2005 period, hovering around the 2 percent levelwith very little high-frequency variation; instead, the RW model attributes most of
the variation in the 10-year bond yield over time to changes in the expected future
path of short rates. By contrast, the BRS model attributes most of the high-frequency
variation in the 10-year yield to changes in the term premium component, with therisk-neutral component generally trending smoothly downward over time.
Why is the decomposition implied by the two models so different? Recall that the
BRS model is estimated in two stages, with the macroeconomic VAR estimated first. The
smoothly downward-trending risk-neutral rate in the BRS model is essentially a projection of the future path of short rates based on the VAR, so the VAR in the BRS modelis implying a smoother path for the risk-neutral 10-year yield than is the case for theactual 10-year yield in the data. By contrast, in the RW model, the future path of shortterm interest rates is affected greatly by the inflation “level” factor Lt . Since Lt can varyat high frequency in response to exogenous shocks to itself and to inflation, the RW
model’s specification allows these shocks to pass essentially directly through to therisk-neutral 10-year yield, leading to significant high-frequency variation in that variable.
The high-frequency variation in the risk-neutral 10-year yield that is allowed for (butnot assumed) by the RW model is arguably one of its strengths—indeed, Gürkaynak,
Sack, and Swanson (2005) found significant systematic variation in far-ahead forward
nominal interest rates in response to macroeconomic news in a way that suggestedchanges in inflation expectations rather than changes in term premiums. Similarly,
Kozicki and Tinsley (2001) found that statistical models that allow for a “moving
endpoint” are able to fit interest rate and inflation time series much better thanstandard stationary or difference-stationary VARs. A weakness of the BRS model and99many other macroeconomic models, according to both of these papers, is that thosemodels assume long-run features of the economy, such as the steady-state real interest
rate and rate of inflation, are too well anchored in response to shocks. The RW model
relaxes that restriction to a much greater extent than does the BRS model.17
Finally, as was the case for the BRS model, the recent episode of low 10-yearyields is one that the RW model conspicuously fails to fit. Thus, from the perspectiveof both the BRS and RW models, the recent behavior of long-term Treasury yields
does represent a conundrum.

IV. Factors That May Underlie the Conundrum
According to the BRS and RW macro-finance models, the recent behavior of long-term
interest rates does present us with something of a conundrum. In this section, we investigate whether any additional factors lying outside of the two models could potentiallyexplain this episode of unusually low long-term bond yields. One could of course searchfor correlations between our macro-finance model residuals and nonlinear functions of
the variables that are already included in the model, such as the output gap, inflation,and the federal funds rate. We focus instead on searching for variables outside of themodels because we do not have particularly strong priors that nonlinearities in theabove variables have played an important role in the U.S. long-term Treasury market
recently, while many plausible candidate variables have been omitted from the model(a number of which have been emphasized in the financial press).
We first briefly review various explanations for the conundrum that have been
discussed in the literature, and then provide some empirical analysis based on theestimated macro-finance models to help assess the importance of the various factors.
A. Popular Explanations for Low Long-Term Bond Yields
A large number of possible candidate explanations for low bond yields have been
discussed by financial analysts. A simple means to summarize this set of explanations(and to some extent limit consideration to the most important ones) is to examine asurvey of bond traders, hedge fund managers, and business economists conducted by
the firm Macroeconomic Advisers. Their survey was taken in early March 2005 andasked participants to provide their views on the importance of various factors thatmight explain the low level of the 10-year Treasury yield. Table 5 displays the sevenmost important factors that were identified as holding down bond yields as well as arough estimate of how much each factor was judged by survey respondents to have
reduced the bond yield (in basis points).18 The largest effect by far was attributed toincreased demand for U.S. longer-term securities from foreign central banks. Indeed,
17. The VAR in the BRS model is not restricted to be stationary, and thus the BRS model does allow for an unanchoredsteady state to the extent that the model estimates a unit root. Nonetheless, the estimation of the VAR is basedon the high-frequency dynamics of the system—using four lags of monthly data—rather than on the behavior of
long-term bond yields, and the high-frequency dynamics of the model may not speak so strongly in favor of a unit
root as do long-term bond yields or other more direct measures of the long-term implications of the model.
18. Several other factors (such as increased demand by holders of mortgage-backed securities) were often noted in thesurvey but were assessed to have an effect of only a few basis points and are excluded from the table.

100 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

Table 5 Survey Respondents’ Assessments of Factors Holding Down
the 10-Year Treasury Yield
Factor affecting yields
Demand by foreign central banks
Increased demand by pension funds“Reaching for yield”
Minimal inflation risk
Greater transparency of the Fed
Excess global savings
Low economic growth volatility

Effect in basis points
21
11
10
10
8
8
7

Source: Macroeconomic Advisers (2005).

on average, the survey respondents thought that purchases by foreign central banks
had lowered U.S. long-term rates by 21 basis points.19
After foreign official purchases of U.S. Treasuries, survey respondents assigned
about equal importance to the other factors in Table 5, with each factor accounting
for 7 to 11 basis points of lower long-term rates. For example, an increased demand
for long bonds in light of the greater likelihood of future corporate pension fundreform in the United States, the United Kingdom, and elsewhere was assigned an
importance of about 11 basis points. Such reforms would likely encourage pension
funds to better match the duration of their assets to their liabilities, which is expected
to boost demand for long-term bonds.20 Although this factor has been widely cited(e.g., Bank for International Settlements [2005]), it is difficult to quantify, and we do
not include it in our empirical analysis below.
Similarly, we do not quantify two factors that appear to relate to shifts in investor
appetite for risk. These include the factor, “reaching for yield,” which is shorthand forthe view that high levels of liquidity had encouraged investors to reduce their aversion
to risk (perhaps irrationally in an almost bubble-like manner). Also, a closely related
factor is the view that excess global saving—the global saving glut hypothesis of
Bernanke (2005) noted in Section I—stemming perhaps from less home bias among
foreign investors or rapid economic growth in countries with high saving rates, hadboosted foreign demand for bonds generally (e.g., Warnock and Warnock [2005]).
We do, however, employ some proxies that try to measure a possible decrease in theamount of risk that investors may feel they face. Indeed, the Macroeconomic Advisers
survey respondents pointed to lower uncertainty as an important factor in three of their
responses: minimal inflation risk, greater transparency of the Fed (which presumably
would translate into lower short-term interest rate uncertainty), and low economic
growth volatility. Taken together, these three factors suggest a very important role for
reduced macroeconomic uncertainty in reducing the long-term bond rate.
19. In the academic literature, BRS (2004) and Warnock and Warnock (2005) have also stressed the importance ofthe recent increases in foreign central bank purchases, especially by East Asian countries. Wu (2005), however,notes that finding significant effects of such purchases on U.S. Treasury yields is sensitive to the sample period
and can be quite elusive.
20. These demands for duration would, of course, reach out much longer than a 10-year horizon, and indeed in2005 there was strong demand for the revived 30-year U.S. Treasury bond, for a newly launched French 50-yearbond issue, and for a 50-year British inflation-indexed bond.

101

B. An Empirical Assessment of Various Explanations
To examine some popular explanations for low long-term bond yields, we consider
six variables excluded from the BRS and RW macro-finance models: three measuresof financial market volatility, two measures of macroeconomic volatility, and one
measure of international capital flows. For financial market volatility, we use theMerrill Lynch MOVE Index to measure the implied volatility in the longer-term
U.S. Treasury market;21 we use the implied volatility from Eurodollar options tomeasure uncertainty about the near-term path of monetary policy;22 and we use the
VIX measure of implied volatility from options on the S&P 500 index to measure
uncertainty in the stock market. For macroeconomic uncertainty, we proxy for
output uncertainty by using the eight-quarter trailing standard deviation of thegrowth rate of real GDP, interpolating between the resulting quarterly volatility
measure to get a monthly series, and we proxy for inflation uncertainty by using
the 24-month trailing standard deviation of core PCE deflator inflation. Finally, we
proxy for foreign government and foreign central bank purchases of U.S. Treasury
securities by using the 12-month change in the custodial holdings by the New YorkFed for all foreign official institutions (and we normalize this series by the total stockof U.S. Treasury debt in the hands of the public).23
All of these series are natural candidates for omitted variables that could be impacting the level of long-term U.S. Treasury yields. For example, reduced volatility inlong-term bond markets would tend to make those securities more attractive relative
to other assets and drive long-term bond yields down, all else equal. Conversely, a
reduced degree of uncertainty about stock returns would tend to drive the prices ofstocks up and might require long-term interest rates to rise to keep fixed-income
securities attractive by comparison. Reduced uncertainty about the future path ofmonetary policy may lower the riskiness of holding long-term bonds leading to afall in long-term yields. Reduced uncertainty about inflation may make fixed income
securities of all maturities more attractive, and reduced uncertainty about output
may lower the risk premium and raise the prices of all risky assets in general. Finally,
inelastic demand by foreign central banks and governments for long-term U.S.
Treasury securities has often been cited in the popular press (and in Warnock andWarnock [2005]) as a reason why yields on those securities have been so low recently.
21. The Merrill Option Volatility Estimate (MOVE) Index reports the average implied volatility across a wide range ofoutstanding options on the two-year, five-year, 10-year, and 30-year U.S. Treasury securities (with a total weight of
40 percent on the 10-year Treasury and total weights of 20 percent each on the other maturities). Note that theoptions underlying the MOVE Index have expiration dates of approximately one month; thus, the MOVE Index
measures the implied volatility of long-term yields over a relatively short horizon. Such short-term uncertainty
about long-maturity yields may be related but is not the same as uncertainty about the path of short-term interest
rates in the distant future.
22. Specifically, we use the closest to at-the-money Eurodollar option with expiration in six months’ time to measure
the implied volatility for the 90-day Eurodollar rate.
23. The idea that the available quantity of long-term U.S. Treasury securities matters for long-term yields suggests that
we should also look at total issuance of marketable U.S. Treasury securities by the U.S. government as well as thepurchases of those securities by foreign governments. In preliminary results not reported here, we do find evidence
that greater total issuance of marketable U.S. Treasuries does lead to higher long-term interest rates (Bikbov andChernov [2006] also find a correlation between total issuance of U.S. Treasuries and their macro-finance model
residuals). Even when we include total issuance as an explanatory variable in our regressions below, however, we findlittle evidence that foreign official purchases have had a significant impact on long-term Treasury yields.

102 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

Graphs of each of these six variables over the 1984–2005 sample are presented in
Figure 3 (note that not all of the above series, particularly the implied volatility series
for financial market data, are available for the early part of the sample). It is immediately clear from the figure that many of these series show a marked dip in the last few
years—thus, as readers of the financial press are well aware, there is no shortage ofpossible explanations for the conundrum!
Figure 3 Explanatory Variables for BRS and RW Model Residuals[1] Implied Volatility on Longer-Term
Treasury Securities (MOVE Index)[2] Implied Volatility on Six-Month-Ahead
Eurodollar Futures (From Options)

160

200

Basis points per six months

175

140

150
120

125

100

100
75

80

50

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
9
20 9
00
01
02
03
04
05

25

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
9
20 9
00
01
02
03
04
05

60

[3] Implied Volatility on S&P 500 (VIX)[4] Realized Volatility of GDP Growth(Trailing Eight Quarters)

35

5

30

4

25

3

20

2

15

1

10

0

Standard deviation, annualized percentage points

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
20
00
01
02
03
04
05

6

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
9
20 9
00
01
02
03
04
05

40

[5] Realized Volatility of Monthly Core
PCE Inflation (Trailing 24 Months)[6] Foreign Official Purchasesof U.S. Treasury Securities

0.18

Standard deviation, monthly percentage points

0.16

8.0

Percentage of outstanding debt

6.0

0.14

4.0

0.12
2.0
0.10
0.0

0.08

–4.0

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
9
20 9
00
01
02
03
04
05

0.04

19
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
9
20 9
00
01
02
03
04
05

0.06

–2.0

Note: Not all data series going back to 1984 are available.

103

In Table 6, we conduct a preliminary statistical analysis of the importance of thesevariables by regressing the residuals from the BRS model and the RW model on eachof the six independent variables described above, in a set of univariate regressions.
A significant correlation between one of these variables and the models’ residuals
would suggest that that variable is perhaps an important determinant of long-term
U.S. Treasury yields that has not been captured by the model.24
As can be seen in Table 6, and as was evident in Figure 3, many of the variables
above are correlated with the models’ residuals. Indeed, five of the six variables
are statistically significant at the 10 percent level for at least one of the two models’
residuals. Two of the six variables—implied volatility on long-term Treasuries and
GDP volatility—are statistically significant at the 5 percent level or better for bothmodels’ residuals. Interestingly, foreign official purchases of U.S. Treasuries—thevariable that is most emphasized by the financial press and by the Macroeconomic
Advisers survey respondents—is the only one of the six variables that is not significant
at even the 10 percent level for either of the two sets of model residuals (and has apositive sign, rather than the negative correlation hypothesized by the press). This
result is very similar to that of Wu (2005), who finds that, after controlling for macroeconomic determinants of bond yields, the correlation between foreign official
purchases and bond yields is significantly positive between 1987 and 2000, andnegative only since 2002. Thus, the correlation that has been emphasized so much by
the financial press is not one that has been consistent over recent history.
Table 6 Univariate Regressions of BRS and RW Model Residuals
Independent variable
Implied volatility on longer-term Treasury securities(Merrill Lynch MOVE Index, in basis points)
Implied volatility on six-month-ahead Eurodollar
futures (from options, in basis points)
Implied volatility on S&P 500 (VIX)
Realized volatility of GDP growth (trailingeight-quarter standard deviation, in percent)Realized volatility of monthly core PCE inflation(trailing 24-month standard deviation, in percent)Foreign official purchases of U.S. Treasury securities (trailing
12-month total, as percentage of U.S. debt in public hands)

Model
BRS
RW
BRS
RW
BRS
RW
BRS
RW
BRS
RW
BRS
RW

Sample
4/88–12/05
4/88–12/05
1/89–12/05
1/89–12/05
1/90–12/05
1/90–12/05
1/88–12/05
1/88–12/05
1/88–12/05
1/88–12/05
5/90–12/05
5/90–12/05

Coefficient t-statistic
1.10
7.83
0.38
4.94
0.278
3.17
0.042
0.95
0.814
1.85
0.049
0.23
20.3
4.25
5.7
2.26
316
2.33
99
1.41
149
0.89
47
0.58

R2
0.23
0.10
0.05
0.00
0.02
0.00
0.08
0.02
0.02
0.01
0.00
0.00

Note: BRS and RW model residuals are in basis points. Sample period start dates differ owing todifferences in data availability for each explanatory variable. Each regression also includes
a constant term (not reported).
24. The purpose of these regressions is to help identify which variables might potentially be the most important toincorporate into the models going forward. Of course, just because a variable is significantly contemporaneously
correlated with our macro-finance model residuals does not necessarily imply that it will remain significant when
fully incorporated into a macro-finance framework, although we interpret a strong contemporaneous correlation
as suggestive that it will be. Also, there is no guarantee that our macro-finance model residuals are orthogonaleven to the variables included in the models because of the overidentifying restrictions those models impose. We
interpret the regression results under the assumption that the overidentifying model restrictions are correct,
although again, we consider both models in order to provide some assurance of robustness.

104 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

A primary problem with Table 6, of course, is that many of the variables show
declines in 2004 and 2005, and thus univariate regression results may be doublecounting the explanatory power of the variables for the conundrum. Table 7 thus
reports results from multivariate regressions for the same variables over the May 1990
to December 2005 period (the longest sample for which we have data on all six
explanatory variables).
The multivariate regression results in Table 7 are, for the most part, consistent withthe univariate regression results in Table 6. The most significant and robust explanatory variable is the implied volatility on longer-term Treasuries. Macroeconomic
volatility also seems to play a statistically significant role, with core PCE inflation
volatility significant at the 5 percent level for both models’ residuals, and GDP growthvolatility significant for the BRS model residuals. The signs of these coefficients arealso what one would typically expect, with lower implied volatility on Treasuries andlower realized volatility of output and inflation all being correlated with lower yields
on long-term Treasury securities. Again, the series on foreign official purchases is the
only one not statistically significant at the 10 percent level for at least one of the twomodels’ residuals and continues to have a positive sign.
C. Decomposition of the Bond Yield Conundrum
How much of the bond yield conundrum can these additional variables explain? In
Table 8, we perform a detailed decomposition of the decline in bond yields from June
2004 to June 2005, according to each of the BRS and RW models and the regression
results from Table 7. We choose June 2004 as the starting point for the change in yields
because that month, when the FOMC embarked on the removal of monetary accommodation, is most often cited as the beginning of the conundrum. During this period,
long-term interest rates edged down as the Fed steadily tightened policy.
The first row of Table 8 reports the actual change in 10-year yields over the June
2004 to June 2005 period (which differs slightly across the BRS and RW models
because the BRS model uses month-average yield data while the RW model uses
Table 7 Multivariate Regressions of BRS and RW Model Residuals
Independent variable
Implied volatility on longer-term Treasury securities(Merrill Lynch MOVE Index, in basis points)
Implied volatility on six-month-ahead Eurodollar
futures (from options, in basis points)
Implied volatility on S&P 500 (VIX)
Realized volatility of GDP growth (trailingeight-quarter standard deviation, in percent)Realized volatility of monthly core PCE inflation(trailing 24-month standard deviation, in percent)Foreign official purchases of U.S. Treasury securities (trailing
12-month total, as percentage of U.S. debt in public hands)
R2

BRS model
RW modelCoefficient (t -statistic) Coefficient (t -statistic)
1.203 (5.47)

0.490 (4.11)–0.229 (–1.35)–0.168 (–1.83)–0.334 (–0.63)–0.497 (–1.73)

15.4(3.10)

3.9

360(2.18)

214(2.39)

147(0.76)

38(0.04)

0.30(1.45)

0.14

Note: BRS and RW model residuals are in basis points. Sample is May 1990 to December 2005.
Each regression also includes a constant term (not reported).

105

Table 8 Decomposition of Long-Term Bond Yield Conundrum
BRS model

RW model

–93.3

–87.0

13.1
–19.9
–86.5

–29.6
–25.2
–32.2

–29.9
1.1
–11.6

–12.2
0.7
–2.9

7.0
1.2
–6.0
–48.6

5.1
1.7
–1.6
–23.0

Observed change in 10-year yield (basis points), June 2004–
1
June 2005
Of which:
2 Model-implied change in risk-neutral 10-year yield
3 Model-implied change in term premium
4 Change in model residuals
Of which:
5 Change in implied volatility on longer-term Treasuries
6 Change in realized volatility of core PCE inflation
7 Change in realized volatility of GDP growth
8
9
10
11

Change in implied volatility of Eurodollar rate
Change in implied volatility of S&P 500
Change in foreign official purchases
Unexplained by above

Note: Row 1 denotes the change in observed zero-coupon 10-year Treasury yields from June 2004
to June 2005; the values differ across the BRS and RW models because the BRS model uses
month-average yield data while the RW model uses end-of-month data. Rows 2–4 are thedecompositions implied by the BRS and RW macro-finance models, graphed in Figures 1 and 2.
Rows 5–10 are the changes implied by the regression coefficients in Table 7 applied to thechange in each independent variable from June 2004 to June 2005. Row 11 is the difference
between row 4 and the sum of rows 5–10.

end-of-month yields), amounting to a fall of about 90 basis points. Both the BRS and
RW models imply that only a small part of this change—about 20 to 25 basis points—
can be attributed to changes in the term premiums (row 3). Moreover, the BRS modeleven predicts that risk-neutral long-term yields should have risen about 13 basis points(row 2), based on the improving outlooks for GDP and inflation. Thus, the decline
in long-term yields from June 2004 to June 2005 is largely unexplained by both the
BRS and RW models, with changes in the models’ residuals (row 4) accounting forabout 87 and 32 basis points of the roughly 90-basis-point fall in yields from June
2004 to June 2005.
Rows 5–10 of Table 8 decompose the BRS and RW model residuals into the parts
that are explained by each of the six variables in the previous section, according to
the regressions in Table 7 multiplied by the change in each independent variable over
the June 2004 to June 2005 period. Not surprisingly, the fall in implied volatility
on longer-term Treasuries from June 2004 to June 2005 accounts for the greatest
fraction of the two models’ residuals, explaining a little over one-third of the residualsof each model. More surprisingly, the change in core PCE deflator inflation volatility
over this period accounts for essentially none of the conundrum, even though it
was statistically significant in Table 7. Apparently, the change in inflation volatility
from 2004 to 2005 was close to zero and even slightly positive, so that the conundrum period played little role in the statistical significance of the results for thatvariable in Table 7. The fall in GDP volatility from 2004 to 2005 seems to haveplayed a more important role, accounting for about one-eighth (12 basis points)of the BRS model residuals and about one-10th (3 basis points) of the RW model

106 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

The Bond Yield “Conundrum” from a Macro-Finance Perspective

residuals. The remaining three variables in Table 8 were not statistically significant
in the previous section and account for relatively small changes in yields over theconundrum period.
As can be seen in the last row of Table 8, even the six omitted variables studied in
the previous section can explain only about one-fourth to, at most, one-half of the
BRS and RW model residuals. Thus, we have by no means found the missing linkthat explains the bond yield conundrum, but we have found evidence that reductions
in longer-term Treasury volatility probably have played an important role, and that
foreign official purchases of U.S. Treasuries probably have not.

V. Conclusions
We draw a number of conclusions from the above analysis. First, the low level oflong-term bond yields in the United States during the 2004–05 period does appear
to be a conundrum when viewed through a macro-finance lens. Specifically, neitherof the two macro-finance empirical models we consider is able to explain the recent
low level of, or the fall in, long-term bond yields. This finding is remarkable giventhat both models fit the earlier long-term yield data quite well. Therefore, the conundrum can likely only be explained with variables that lie outside of our baseline
macro-finance models. Of the six such variables that we consider, declines in thevolatility of long-term Treasury yields seem to have played the most important role.
Even so, at best, almost two-thirds of the conundrum remains unexplained.
Interestingly, we find that the explanation for the conundrum emphasized byfinancial market participants—namely, large-scale purchases of long-term Treasuries byforeign central banks—has essentially no explanatory power for the conundrum episode.
This discrepancy may reflect a difference between unconditional and conditional correlations. In particular, long-term Treasury yields have been declining steadily over time,and foreign official holdings and purchases of U.S. Treasuries have been rising steadily;
thus, the unconditional correlation suggests a substantial negative effect. In contrast, amacro-finance econometric framework attributes the downward trend in long-termyields largely to declining current and future projected levels of inflation. After controlling for such factors, the residuals from our two baseline macro-finance models
have no significant correlation with foreign official purchases of U.S. Treasuries.
Of course, this leaves us with about two-thirds of the bond yield conundrum yet
to be explained, to say nothing of the similar or perhaps even more extreme behaviorof long-term yields in other countries, such as Germany and Japan. The resolution ofthese “conundrum” episodes, in the United States and abroad, presents a rich frontier
for future research.

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Comment
CARSTEN DETKEN25
European Central Bank

I. Introduction
I would first like to thank the organizers for inviting me to discuss this interesting andvery topical paper by Glenn D. Rudebusch, Eric T. Swanson, and Tao Wu.
My comments are structured in four parts. In Section II, I would like to start withsome general remarks on the macro-finance literature in general and on the Rudebusch
and Wu (RW [2004]) model in particular. In Section III, I will use a very similar
macro-finance model developed by my European Central Bank (ECB) colleagues
Peter Hördahl (now at the Bank for International Settlements), Oreste Tristani, andDavid Vestin estimated for the euro area, and reveal that they also find that current
bond yields—the German seven-year bond yield, to be precise—are too low comparedto the prediction of their state-of-the-art, macro-finance, term structure model. In
Section IV, I will then try to explain the possible euro-conundrum by the very same
25. The views expressed are solely my own and do not necessarily reflect those of the European Central Bank. I thankmy colleagues Oreste Tristani and David Vestin for sharing valuable insights about the Höhrdahl-Tristani-Vestinmodel and Oreste Tristani for providing an updated residual series of their model. I also thank Ramon Adalid,
Björn Fischer, and Marco Lo Duca for providing the other data used in this comment and Michaela Kloepfer for
editorial support.

109

model-exogenous variables, as the authors of the current paper have done. The only
variable able to account for the euro-conundrum is a measure of euro area “excess
liquidity.” Therefore, finally, in Section V, I will discuss the possible role of excess
liquidity, in particular, whether it could be considered an explanatory factor or simply
another measure of the same phenomenon.

II. The Macro-Finance Literature and the RW Model
To speak about an interest rate conundrum implies having a specific model in mind
that defines the benchmark for bond yields. Pure finance models are able to explain
current low long-term bond yields with ease (see Kim and Wright [2005]). Moreover,there are even ample explanations as to why the yield curve could actually be negatively
sloped in equilibrium due to the hedge long bonds provide against adverse consumption outcomes; see, for example, Campbell (1986) or Piazzesi and Schneider (2006).
Empirically, there have been at least three other episodes where U.S. long-term bondyields declined after the first 100-basis-point hikes in short-term interest rates: in1971, 1976–80, and 1983–84; see Credit Suisse (2006).
The use of the term conundrum thus only makes sense if attached to a specificmodel class. In the current paper, the authors chose the macro-finance class of models,
and that is an excellent choice. The macro-finance literature is a most promisingbranch of the literature, and we have seen an important evolution in the last few
years. Furthermore, the present authors have been at the frontier of this evolution,
together with Ang, Piazzesi, Diebold, Li, and Hördahl, Tristani, and Vestin (HTV).
The evolution we have seen has led to important improvements, because these models
allow for time-varying risk premiums in a no-arbitrage consistent manner. The risk
premiums depend on macroeconomic conditions, and mutual feedback between themacroeconomy and interest rates is allowed for.
The RW and HTV type models in the macro-finance literature usually describe
the macroeconomy using a standard new Keynesian setup. They use a Phillips curve,
an intertemporal Euler equation (“IS” curve), and a monetary policy rule (Taylortype rule). The time-variable prices of risk are then determined as linear functions of
the state of the economy. The prices of risk then translate one unit of factor volatility(which itself is constant over time) into a risk premium for each maturity, fully
respecting the no-arbitrage conditions between different maturities.
In the RW model, the short rate is a function of two latent variables: the mediumterm inflation goal and the Taylor rule. Four particular features of the RW model
deserve to be mentioned. First, the policy instrument in the Taylor rule is defined interms of the slope of the yield curve, not the short-term interest rate (one could ofcourse ask whether the slope of the curve is really under the control of the central bank).
Second, there is no forward-looking inflation term in the Phillips curve and, third, thereal interest rate in the Euler equation is not derived with expected inflation but withthe medium-term inflation goal. Fourth, the prices of risk are only functions of the twolatent variables but not directly inflation and output. These choices might appear
peculiar, but I have no evidence that they bias the results in any particular direction.

110 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

The second macro-finance model the authors use to assess the conundrum is the
Bernanke-Reinhart-Sack (BRS [2004]) model, which describes the macroeconomy bymeans of a five-variable VAR instead of a structural model. This model uses only
observable and no latent variables, so that the larger residuals compared to the RW
model are no surprise. What is a bit surprising is the different size and even sign ofresiduals for some periods obtained after the authors reestimated the BRS model toinclude data up to December 2005. For example, the originally negative residuals in2003 of about 150 basis points (BRS [2004, figure 9]) turned into positive residualsof about 50 basis points in the same period with the updated estimation. I will notcomment further on the BRS model, as the RW model is more similar to the HTV
model used later for comparison.
The macro-finance literature delivers some important results. Risk premiums are
anti-cyclical, as higher risk premiums are requested in bad times. It follows that apositive output shock can lead to a fall in long-term government bond yields. This is
the case when the negative effect on the risk premium is stronger than the positiveeffect on the risk-neutral rate. HTV (2006) have also shown that the HTV model has
superior forecasting power for yields at all maturities. The reason is that macro-financemodels can isolate the pure expectations component of future interest rates. They findfor the euro area that the reduction in the average forecast error is 20 percent compared
to a random walk, 40 percent compared to a pure finance model, and 100 percent withrespect to a VAR.
The macro-finance literature is still evolving, as progress is slow. These models arecomputationally very demanding, and some shortcuts are needed simply to allow
tractability. There is as yet no direct effect from long interest rates and risk premiumson the macroeconomy. Only the short rate affects the economy. Then there are someresults that are indeed puzzling at first sight, for example, that an increase in the
inflation target lowers the risk premiums for longer maturities while it increases themfor shorter maturities. This is possibly because investors believe that the inflation
target itself is mean reverting. Alternatively, lower long bond risk premiums could
result (since the 1990s) from a positive correlation between inflation and consumption, so that a rise in inflation lowers risk premiums, because consumption is also
expected to be higher in the future (see Piazzesi and Schneider [2006]). And finally, themicro-foundations of these models are not yet fully satisfactory. The pricing kernel,that is, the stochastic discount factor, should endogenously depend on consumption
preferences, while in these models it is constant.

III. Conundrum or Conundra?
After these general remarks, we will investigate the performance of a macro-financemodel in the euro area. Figure 1 shows evidence that using the macro-finance modelby HTV (2006) estimated for German bond yields and euro area macro data
between 1991 and 200626 reveals a similar conundrum for the euro area as for the
26. The model allows for a structural break in the monetary policy rule in 1999/I.

111

Figure 1 HTV Macro-Finance Model for the Euro Area: A Euro-Conundrum?
Basis points

Percent
Macro-finance residual (left scale)Seven-year German government
bond yield (right scale)
Macro-finance fitted (right scale)

10

8

0.8

6

0.4

4

0.0

2

–0.4

–0.8
1991

93

95

97

99

2001

03

05

United States, although for the euro area there was one other period with similarly
large residuals, in 1998. Given financial market integration, perhaps not surprisingly
we can establish at least two conundra. The HTV model is very similar to the RW
model, although it avoids those four rather special features mentioned in the previous
section and is in this sense more standard in its macroeconomic setting. The results
shown here for the period of European Monetary Union (EMU) are very preliminary, however, as one might guess from the fact that the residual series still seems
to incorporate some autocorrelation. There is only one unobservable variable: theinflation target. HTV (2006) find that the risk premium on long-maturity bonds
during the EMU period is mainly due to the inflation target risk and output risk,rather than the interest rate risk and inflation risk.
Interestingly, Figure 1 shows that the negative residual in German bond yields inthe last two quarters where data were available (2005/IV and 2006/I) is due to thefact that the HTV model predicts a faster rise in yields than has actually happened.
The changes in the macro-finance residual reported by Rudebusch, Swanson, andWu (2006) for the U.S. 10-year government bond between June 2004 and June 2005
(the latest observation period in their sample) amount to –86.5 basis points in the
BRS model and –32.2 basis points in the RW model. As depicted in Figure 1, theHTV residual reports a comparable residual change for the German seven-year bondof about –61 basis points between 2004/II and 2006/I.
Figure 2 portrays the same decomposition of the HTV fitted German yield into
the risk-neutral rate and the term premium as the authors have for the BRS and RW
models in their paper.
Similar to the RW model results, the term premium is relatively constant, whilechanges in the fitted rate are driven by the risk-neutral rate, which is a standard result
for these models.

112 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

Figure 2 Decomposition of Fitted Rate in Risk-Neutral Rate and Term Premium

9Percent
Macro-finance fitted
Risk-neutral rate
Term premium

8
7
6
5
4
3
2
1
0
1991

93

95

97

99

2001

03

05

IV. Explaining the Euro-Conundrum
Rudebusch, Swanson, and Wu (2006) attempt to explain the residual using the “usual
suspects” (derived from survey respondents’ assessment of factors holding down longterm bond yields as of March 2005). The possible factors mentioned in the survey aredemand by foreign central banks, increased demand by pension funds, reaching foryield type of behavior, a very low inflation risk, greater transparency of the Federal
Reserve, excess global savings, and low volatility of economic growth. The authors
quantify some of these factors and regress the model residuals on these variables(both univariate and multivariate regressions are used). They use implied volatility on
longer-term Treasury securities, implied volatilities on six-month-ahead Eurodollar
futures, implied volatility on the S&P 500, realized volatility of quarterly GDP growth,realized volatility of monthly core inflation, and foreign official purchases of U.S.
Treasury securities as a percentage of U.S. debt in the hands of the public.
In one sense, the approach to regress the model residual on an easily available timeseries is inconsistent with this model class, which takes pride in allowing no arbitrage
opportunities. On the other hand, the quantity of risk, as mentioned above, is at thisstage constant in these models. So the idea to especially try to explain the residuals by
variations in volatility of some of the factors is very reasonable (taking for granted thatthe first-best solution to include these exogenous variables directly in the model is
intractable at this stage). Obviously, there is also a risk of reverse causality for someof those variables, for example, the low bond yield could attract foreign purchases due
to a weak U.S. dollar, or small figures for bond yields could lead to low measured

113

volatilities. But I will abstract from such issues here, as they are likely not to be crucial
and simply perform the same exercises suggested by the authors for the German
seven-year bond yield residual. The conclusions from Rudebusch, Swanson, and Wu(2006) have been that implied volatility of bond yields and to some smaller extent
volatility of GDP growth can explain part of the conundrum. But the authors argue
that the major part of the residuals still remains unexplained (48.6 basis points for the
BRS model and 23.0 basis points for the RW model). They thus conclude that there
really has been a conundrum, at the end of their sample period in mid-2005.
Let me include one remark on the authors’ dismissal of the official foreign purchases hypothesis. The data the authors use to measure official foreign purchases
are the custodian holdings by the New York Fed for all foreign official institutions(normalized by the U.S. stock of Treasury debt in private hands). This measure declined
strongly from fall 2004, thus during the heydays of the conundrum, which is why
this variable hardly contributes to explaining it. But there is anecdotal evidence that
there has been a widening gap between foreign official purchases of U.S. Treasuries
and total foreign purchases of U.S. Treasuries due to the reinvestment of OPEC
oil revenues. OPEC investments are often not counted as official purchases by theFed, because they are frequently invested in U.S. Treasuries via third parties in
the City of London and Caribbean banking centers. It is thus possible that a temporary foreign demand factor contributed more to the conundrum than is acknowledgedin the paper.
I then use similar measures for some of the “usual suspects” as Rudebusch,
Swanson, and Wu (2006) to see how much these factors could explain the depictedeuro-conundrum between 2004/II and 2006/I. The macro-finance residuals have been
regressed on the Euro Interbank Offered Rate (Euribor) volatility, option-implied
10-year bund future (one-month) volatility, euro area real GDP growth volatility, euroarea inflation volatility, and two measures of euro area excess liquidity. I tried both
univariate as well as multivariate regressions, both in levels (allowing for autocorrelated
residuals of order one) and in first differences, with and without a dummy for the
EMU period since 1999 (before that, the euro area data are synthetically constructedfrom national data). The measures of excess liquidity will be defined below, while theother variables are defined as in Rudebusch, Swanson, and Wu (2006). In contrast tothem, I use quarterly data as opposed to monthly data for this exercise.
Only implied bond yield volatility and the measures of excess liquidity had any
statistically significant influence on the residual. I will concentrate on these twovariables in the following.
Figure 3 plots the implied 10-year bund future volatility and the macro-financeresidual. The vertical line indicates 2004/II, the time at which the authors date thestart of the conundrum period. While in general Figure 3 visually confirms theregression results that bund future volatility is somehow positively correlated with theunexplained part of the macro-finance model, it is also evident that there has been no
further decline in volatility in the period since 2004/II. It is thus not surprising thatfor the decomposition of the conundrum shown below, and in contrast to the results
for the United States, by Rudebusch, Swanson, and Wu (2006), volatility contributed
hardly anything to solving the puzzle of low bond yields.

114 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

Figure 3 Can Low Bond Volatility Explain the Euro-Conundrum?

0.6

Basis points

Percent
Macro-finance residual (left scale)
Implied 10-year bund future
volatility (right scale)

0.4

9

8

0.2

7

0.0

6

–0.2

5

–0.4

4

–0.6

3
1991

93

95

97

99

2001

03

05

The only other variable that turned out to be significant in the regression analysis
was euro area excess liquidity. I used two measures of excess liquidity, which are
regularly computed at the ECB, the monetary overhang and the real money gap, bothcorrected for portfolio shifts.
The monetary overhang and the real money gap are defined as follows:
overhangt = (m3t − pt ) − (c + yt −  (i t − i M 3,t )),real money gapt = (m3t − pt ) − (c + y * −  (i * − i M* 3)),where m3 is the M3 monetary aggregate, p stands for the consumer price level, y forreal GDP, and i is the nominal short-term interest rate, while i M 3 is the own interest
rate of assets included in M3. All variables are in logs, except the interest rates. c, ,and  are estimated coefficients from a money demand equation. An asterisk highlights equilibrium values as opposed to time t-variables. While the overhang depicts
the current real money balances in excess of money demand given current income andinterest rates, the real money gap provides similar information, but money demandis measured by using estimated equilibrium income and equilibrium interest rates.27
Figure 4 shows that the typical correlation between the macro-finance residual andthe monetary overhang is negative, except for the 1992–93 Exchange Rate Mechanism(ERM) crisis and the following year, as well as sometime in 2002–03. The positive
correlation for the ERM crisis period is most likely due to money demand being
27. See Masuch, Pill, and Willeke (2001).

115

Figure 4 “Excess Liquidity” and the Euro-Conundrum

0.8

Basis points

Percent
Macro-finance residual (left scale)
Monetary overhang (corrected) (right scale)

0.6

10

8

0.4

6

0.2

4

0.0

2

–0.2

0

–0.4

–2

–0.6

–4
1991

93

95

97

99

2001

03

05

Note: The shaded area depicts the pre-EMU period.

Table 1 Decomposition of the Euro-Conundrum
Contribution to the change in the macro-finance residual of seven-year German bond yield between
2004/II and 2006/I of –61 basis points
Implied volatility
Monetary overhangReal money gap
Full sample, levels
–4
–31
–34
Full sample, first difference
–4
–22
–27
EMU dummy, levels
–4
–41
–40
EMU dummy, first difference
–3
–59
–48

underpredicted during the crisis period. The reason was that the average euro areainterest rate used in the computation of money demand had been increased bysome countries defending the exchange rate peg toward Germany by substantiallyhiking short-term policy rates. The fitted low money demand probably does notreflect true euro area money demand at the time. Anyway, there is the issue of theappropriate estimation period for the excess liquidity measures. Below I will reportresults for the whole period and also include an EMU period dummy that is interacted with the excess liquidity measures, and which then focuses on the non-shaded
sample period.
Table 1 reports the results of the decomposition of the three variables that turned
out to be statistically significant. The estimated coefficients are multiplied with thechange in the respective variable over the period from 2004/II to 2006/I to examine
the contribution to the widening of the macro-finance residual by 61 basis points

116 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

over this horizon. The monetary overhang and the real money gap are used alternatively but jointly with the implied volatility measure. Regressions were run inlevels (with AR(1) terms if needed) and in first differences. The EMU dummy row
shows results where the excess liquidity measures have been interacted with a dummy
equaling one for the EMU period and zero otherwise.
The results in Table 1 come not surprisingly after having studied Figures 3 and 4.
Implied volatility, despite being statistically significant over the whole period,
contributes very little to explaining the conundrum, that is, it explains only 3–4 of
the 61 basis points. Even if one starts the decomposition in 2003/III, just before thelarge drop in volatility (see Figure 3), the contribution would only be 10 basis points.
The excess liquidity measures instead explain either about half, or 22–34 basis points,
if the whole sample is used, or nearly all of the conundrum if only the EMU sample
is used, that is, 40–59 basis points. Obviously the interpretation of this finding withrespect to the excess liquidity measures deserves some comment. Depending on
which of the differing views, discussed in the next section, is taken, the above results
show that the euro-conundrum is either fully or not at all solved.

V. The Role of Excess Liquidity in Explaining the Conundrum
Interestingly, the notion of excess liquidity does not receive much attention in theacademic literature, probably due to legitimate conceptual difficulties. However,beyond the academic world it is widely discussed, with a broad spectrum of theories
covered. For example, browsing the Internet, one finds at one extreme articles on thedismal effects of monetary liquidity and excess money at the Ludwig von Mises
Institute (a libertarian think tank) and at the other extreme a discussion of causes
and effects of recent excess savings on the World Socialist Website, besides several
investment bank publications on the topic.
The real issue is whether our measures of excess liquidity could provide at least to
some degree an exogenous explanation of the bond yield conundrum or whether themeasures of excess liquidity and the interest rate conundrum are simply different sidesof the same coin. Exceptionally low yields show up in a negative macro-finance residual
and simultaneously produce positive monetary overhangs and large real money gaps.
I will not be able to resolve this issue, but would like to provide some arguments
why a quick and easy dismissal of the hypothesis that excess liquidity could play animportant role in driving asset prices is premature. There are at least four arguments
to consider.
First, the measures of euro area excess liquidity used above are corrected for
estimated portfolio shifts in and out of M3, which are likely to be only reflections of
temporary changes in liquidity preferences (see European Central Bank [2005]). Thismeans that any aggregate increase in uncertainty—which would lead on the one handto strong demand for safe government bonds and possibly to a negative macro-financeresidual and on the other hand fuels excess liquidity via strong demand for assets
included in M3—would most likely not be the reason for the negative correlation
observed in the right half of Figure 4. Such a portfolio shift would be subtracted from117

the M3 aggregate used here. The ECB uses different indicators to form a judgmenton the existence of portfolio shifts such as the net purchases of non-monetary assets by
the money holding sector and the correlation of equity and long-term bond returns.
The correction might not, of course, be perfect.
Second, let us assume that low short-term interest rates usually lead to strong excess
liquidity. The correlation depicted in Figure 4 could then be the result of a mechanicalrelation between the macro-finance residual and the long bond yield (assuming
the yield curve would move in parallel). Figure 5 shows that this is not the case. The
correlation between the macro-finance residual and the level of the seven-year German
bond yield, as revealed in Figure 5, has only been +0.18 over the whole sample.
Third, the interest rates used in the computations of the overhang and real gap
measures are short- to medium-term rates, while the macro-finance residuals refer tolong-term bond yields. In practice and in theory, there is no mechanical link between
short and long rates. The slope of the yield curve is endogenous in reality as well as in
the macro-finance models. Thus, there is no reason to suspect that periods leading toexcessive monetary growth would systematically and mechanically be associated withexceptionally low long-term bond yields.
Fourth, there is other empirical evidence that during asset price boom periods
measures of excess liquidity (derived by extracting monetary shocks by means of VARs)
contribute to explaining asset price dynamics (especially real residential real estate
prices; see Adalid and Detken [2006]).
Still, even if one accepts that contemplating the possible role of excess liquidity isworth further effort, one of the open issues would be what actually determines thephenomenon of excess liquidity.
What is the final verdict on the euro-conundrum? Recently, we have observed astrong and negative correlation between euro area measures of excess liquidity and the
Figure 5 Correlation between Yield Level and Macro-Finance Residual

0.8

Basis points

Percent
Macro-finance residual (left scale)Seven-year German government
bond yield (right scale)

0.6

9
8

0.4

7

0.2

6

0.0

5

–0.2

4

–0.4

3
2

–0.6
1991

93

95

97

99

2001

118 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

03

05

Comment

unexplained part of long-term German bond yields: the macro-finance residual. The
degree to which one accepts excess liquidity to be an exogenous factor (or the degree towhich it is itself explained by a factor that does not systematically and simultaneously
also determine the long rate, such as, for example, the stance of monetary policy) canmore or less account for the conundrum. To the degree that this is not the case, one
would report the finding of a similar euro-conundrum in 2006/I that Rudebusch,
Swanson, and Wu (2006) have reported for the United States in 2005/II. Casual
observation of broad money growth in the United States suggests that there could bea similar issue with excess liquidity for the United States as for the euro area. Further
research is certainly needed to explore these topics in more depth.
Finally it should be noted that at the time of the conference, the U.S. 10-yearbond yield had increased by 100 basis points to about 5.00 percent from the levels
reported by the authors for 2005/II. Similarly, the German seven-year bond yield had
increased since 2006/I by 25 basis points to 3.75 percent. As of mid-July 2006, there
has been another 15-basis-point increase in the respective German bond yield. Of
course, the current fitted values of the macro-finance models are not available at thepresent stage. But given the small size of the conundra, that is, 61 basis points for theGerman yield and 86.5 basis points (BRS) and 32.2 basis points (RW) for the UnitedStates, of which only 48.6 basis points (BRS) and 23.0 basis points (RW) had
remained unexplained, most likely the conundra had actually disappeared by the timeof the conference.

References

Adalid, R., and C. Detken, “Asset Price Booms and ‘Excess’ Liquidity,” paper presented at the SUERF
Seminar “The Adoption of the Euro in the New Member States: Challenges and Vulnerabilities
on the Last Stretch,” May 4–5, 2006, Malta.
Bernanke, B., V. Reinhart, and B. Sack, “Monetary Policy Alternatives at the Zero Bound: An Empirical
Assessment,” Brookings Papers on Economic Activity 2, 2004, pp. 1–78.
Campbell, J., “Bond and Stock Returns in a Simple Exchange Model,” Quarterly Journal of Economics,
101 (4), 1986, pp. 785–803.
Credit Suisse, “Yield Curve Conundrum,” Quantitative Research, February 1, 2006.
European Central Bank, “Approaches to Identifying and Estimating Portfolio Shifts into and out ofM3,” Monthly Bulletin, January, 2005, pp. 13–16.
Hördahl, P., O. Tristani, and D. Vestin, “A Joint Econometric Model of Macroeconomic and TermStructure Dynamics,” Journal of Econometrics, 131 (1–2), 2006, pp. 405–444.
Kim, D., and J. Wright, “An Arbitrage-Free Three-Factor Term Structure Model and the Recent
Behavior of Long-Term Yields and Distant-Horizon Forward Rates,” Finance and Economics
Discussion Paper No. 33, Board of Governors of the Federal Reserve System, 2005.
Masuch, K., H. Pill, and C. Willeke, “Framework and Tools of Monetary Analysis,” in H. J. Klöckers
and C. Willeke, eds. Monetary Analysis: Tools and Applications, European Central Bank, 2001.
Piazzesi, M., and M. Schneider, “Equilibrium Yield Curves,” paper presented at the 2006 NBER
Macroeconomic Annual Conference, Cambridge, Massachusetts, 2006.
Rudebusch, G., E. Swanson, and T. Wu, “The Bond Yield ‘Conundrum’ from a Macro-Finance
Perspective,” Monetary and Economic Studies, 24 (S-1), Institute for Monetary and Economic
Studies, Bank of Japan, 2006, pp. 83–109 (this issue).
———, and T. Wu, “A Macro-Finance Model of the Term Structure, Monetary Policy, and theEconomy,” working paper, Federal Reserve Bank of San Francisco, 2004.

119

Comment
KENNETH N. KUTTNER
Oberlin College

I. Introduction
The authors’ paper starts with the observation that bond yields remained relatively low
in 2004 and 2005, failing to rise appreciably even as the Federal Reserve increased its
federal funds rate target from a low of 1 percent in mid-2004 to 4 percent by year-end
2005. Many hypotheses have been advanced to explain this phenomenon—which
then-Fed Chairman Alan Greenspan famously labeled a “conundrum”—but this is thefirst paper to rigorously evaluate those hypotheses systematically. It does so using two
state-of-the-art affine term structure models, both of which include macroeconomic
factors, lending the analysis its “macro-finance” perspective. In the end, however,none of the candidate explanations provides an entirely satisfactory explanation forthe phenomenon: Greenspan was right, it really is a conundrum.
The paper is commendable on several dimensions. First, leaving aside anything
it might say about the conundrum specifically, it is a nice application of modern
macro-finance methods, and very revealing about those methods. Its use of two alternative model specifications is also quite informative, and its analysis of the candidate
explanations is impressive in its thoroughness. Finally, the authors are refreshingly
modest in their claims. Negative results, like those in the paper, are useful—and all
too rarely reported.
This comment has three parts. The first part summarizes some key features of themethods used in the analysis, posing some questions about affine term structure
models in general. The second part contains some comments about the model specifications used for the exercise, while the third section sketches another, alternative“lens” for viewing the conundrum. None of this alters the paper’s basic conclusion,
however: the unusually low bond yields in 2004–05 really do represent a conundrum.

II. General Remarks on the Methods
The paper does an excellent job of summarizing the macro-finance methods developed by Ang and Piazzesi (2003), among others, for analyzing the term structure ofinterest rates. It is useful to note, however, that these methods entail two conceptually
distinct elements. The first is a forecasting model for a vector of state variables(call it X t ). Layered on top of the forecasting model is a no-arbitrage asset pricing
model, expressing the bond yields as a function of the underlying state variables.
What makes the whole thing tractable is the assumption that the price of risk, t , isa linear function of the state variables: t = 0 + 1 X t . This assumption allows the bondyields to be written as linear functions of the state variables themselves, with coefficientsdetermined by the 0 and 1 parameters, and by the coefficients in the forecastingmodel. In this context, it is useful to note that if the price of risk is zero (i.e., 0 = 1 = 0),

120 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

then the framework collapses (approximately) to a risk-neutral expectations hypothesis
model, with long-term yields determined (up to a Jensen’s inequality term) by the sumof expected future short-term rates. This feature allows the authors to examine theirmodels’ risk-neutral yields, as well as the implied risk or term premiums.
To implement this approach, the authors choose two “off-the-shelf ” specifications:
one is the VAR used by Bernanke, Reinhart, and Sack (BRS [2004]), and the other isthe small macro model developed by Rudebusch and Wu (RW [2004]). Having fit themodels to the data, the paper confirms the conundrum’s existence by comparing
the fitted yields to the realizations. Then, in a separate analysis, the model residuals
are regressed on proxies representing the candidate explanations. Nothing works: theproxies explain only a fraction of the residuals’ variance.
Except for the puzzling 2004–05 episode, both models do an excellent job offitting the observed yield curve data. And yet, two aspects of the method are vaguely
unsatisfying—or even slightly troubling.
First, it is not at all clear (and the authors provide little guidance) as to what belongsin the state vector X t . The BRS (2004) approach is to throw everything into X t —conventional macroeconomic variables, plus survey measures of inflation expectations
and Eurodollar futures rates. This “kitchen sink” approach makes it hard, if not
impossible, to interpret the 30 estimated 0 and 1 parameters; nor is it clear how one
would account for, in macroeconomic terms, fluctuations in bond risk premiums.
Thus, while the no-arbitrage pricing model imposes some structure on the yield
curve and its evolution over time, the link between the macroeconomy and bondterm premiums is obscure, hidden in a reduced-form “black box.”
With the price of risk depending on only two latent factors, St and L t , determining
the proximate cause of changes in the risk premiums is an easier task, compared with
the BRS model. But these factors capture the level and slope of the term structure,
rather than macroeconomic fundamentals. This use of yields to “explain” yields makes
it difficult to discern the underlying macroeconomic factors affecting the term structure.
A second worrying aspect of the affine term structure methodology is the fact thatthe coefficients relating the bond yields to the state variables are highly nonlinear
functions of the underlying parameters, making estimation “tricky in practice.” Theauthors are commendably candid on this point, noting that the criterion function for
the BRS model is characterized by many local minima—and that they had been able
to improve significantly on the model’s fit, relative to the results originally reported in
BRS (2004). This is probably not a major concern if the sole objective is to maximize“fit” (assuming the global minimum in the 30-dimensional parameter space has been
determined), but it would be more worrisome for questions involving interpretation
or inference.

III. Specific Comments on the Results
Perhaps the most striking feature of the results is that, while both the RW and the
BRS models track the term structure very closely, they have very different implications
for the behavior of the “risk-free” yield (i.e., the forecast of future short-term rates)

121

and the risk premium. The BRS model, with its 30 estimated coefficients in 0 and 1,
fits the long rate quite closely by having a highly variable term premium. The moreparsimonious RW (2004) specification, in contrast, produces a much less volatileterm premium—a nearly constant 2 percent—while the highly volatile fitted “risk-free”
yield gets most of the credit for fitting the observed bond yield.
For the purpose of this paper—which is simply to “fit” the term structure—exactly
how the yield is decomposed into expected future rates and the term premium is ofsecondary importance. In many contexts this breakdown matters, however; policymakers, for one, would surely like to know whether an observed increase in yields
reflects a change in expected future rates, as opposed to a movement in the risk
premium. Both models fit the data fairly well, so one cannot choose between them on
this basis, yet both cannot be correct. The paper’s agnostic treatment of the models is
one of its strengths, but it would nonetheless benefit from a discussion of the criteria
one might use to distinguish between the two specifications.
One attractive feature of the RW (2004) specification is that it allows a “timevarying medium- to long-term inflation rate” to be extracted from the data as thelatent L t . This factor, which appears as expected inflation the IS and aggregate supply
relations, and as the central bank’s inflation objective in the monetary policy rule,
is assumed to depend on its own lag, with an autoregressive coefficient L ; and on
current inflation t , and a random shock L,t .
A question that comes immediately to mind is whether this assumed process
for L t is a reasonable way to model the Fed’s inflation objective. True, research by
Gürkaynak, Sack, and Swanson (2005) suggests that bond yields behave as though
the Fed’s implicit inflation target was a distributed lag of past inflation, with weights
summing to unity. The RW (2004) specification assumes inflation expectations are“unanchored” in the same way: a 1 percentage point positive inflation shock leads to apermanent increase in inflation—and inflation expectations—of roughly 0.33 percent.
This might have been a reasonable description of Fed policy in the late 1980s andearly 1990s—a period characterized by inflation in excess of 5 percent, followed bya period of “opportunistic” disinflation. One has to wonder, however, whether thischaracterization is still appropriate, now that inflation has descended into the range
generally associated with price stability. Or, more to the point: an unmodeled shift
toward a more “anchored” inflation expectations formation process could, perhaps,
explain the apparent conundrum in 2004–05. Further investigation of this hypothesis
is surely warranted.
In a similar vein, explanations involving shifts in the equilibrium real interest ratecannot easily be accommodated in either the RW model (which explicitly includes
the assumption of a constant natural real interest rate) or the BRS model (in which
all variables are assumed to be stationary). Recent research by Laubach and Williams(2003), on the other hand, clearly indicates that the equilibrium real rate has varied
considerably over time. Extending the analysis in a way that allows for equilibrium
real rate shifts seems worthwhile, especially since some of the candidate hypotheses(e.g., the “savings glut”) imply just such a shift.

122 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

IV. An Alternative Conundrum Lens
The affine term structure techniques used in the paper are a powerful tool for understanding the bond yield conundrum of 2004–05. Their disadvantage, however, is thatthey are computation intensive—and as the results illustrate, highly sensitive to model
specification. Examining one-year excess holding returns on the 10-year Treasury note
provides an alternative lens for viewing the phenomenon: while it cannot explain the
cross-section of yields, it provides a way to verify the existence of the conundrum
that is independent of model specification. Conceptually, this approach is comparable
to Hall’s (1978) test of the permanent income hypothesis, whereas the paper’s isanalogous to Flavin’s (1981) use of a VAR to estimate permanent income.
Expressed in terms of excess returns, the expectations hypothesis simply says thatthe expected one-year holding yield on an n-year bond should equal the (known) yield
on a one-year bond—in other words, that the expected one-year excess return on the
one-year bond should be zero (or a constant, departing from the “pure” expectations
assumption). Thus, under the expectations hypothesis, a regression of this excess
return on information known at time t should yield a zero R-squared; a forecastablecomponent to the excess return implies a time-varying risk or term premium.
What is unusual about the 2004–05 episode is that bond yields remained relatively
unchanged, despite the Fed’s campaign to raise interest rates. This suggests that it
would be useful to compare the excess returns over this period with those in othertightening periods. Since the object of analysis is the excess return over the following
year, it makes sense to focus in on the excess returns on bonds held in the 12 months
prior to the first increase in the federal funds rate. These “pre-tightening” periods areshown by the shaded areas in Figure 1, along with the federal funds rate itself.
Figure 2 plots the excess return, relative to a one-year security, on the 10-year notepurchased at the date shown on the horizontal axis, and sold (as a nine-year note)
one year later.28 What is striking is that an investor holding a 10-year note during a“pre-tightening” period would have consistently experienced large, negative excess
returns over the subsequent year. Consistently, that is, until 2004—when that investor
would have earned about the same by holding a 10-year note for one year as he or she
would have by holding a one-year security. Thus, it is the absence of a negative excess
return in 2004 that is the conundrum.
The question, then, is why negative excess returns seemed to have been the predominant pattern until 2004. One hypothesis is that investors were always surprisedby rate hikes, failing to incorporate forecasts of higher short-term interest rates into
bond prices. An alternative hypothesis is that investors knew bond prices would fall—
but willingly accepted below-average (or even negative) returns because of a decline in
the risk premium. This simple approach of examining excess holding returns cannot
distinguish between these two hypotheses, although the fully articulated RW and BRS
models can potentially do so.
However, there is reason to suspect that investors’ ability to forecast policy turning
points has improved in recent years, as the Fed has become more “transparent” with
28. The data are end-of-quarter hypothetical zero-coupon yields derived using the Nelson-Siegel-Svensson (Svensson[1995]) method, and are graciously provided by Eric T. Swanson.

123

Figure 1 The Federal Funds Rate and “Pre-Tightening” Periods, 1984–2006
Percent

12
10

Federal funds rate

8
6
4
2
0
1984

86

88

90

92

94

96

98

2000

02

04

06

Note: Shaded areas indicate “pre-tightening” periods.

Figure 2 The One-Year Excess Return on the 10-Year Bond

50

Percent

40
Excess return on 10-year
U.S. treasury note

30
20
10
0
–10
–20
1984

86

88

90

92

94

96

98

2000

02

04

06

Note: Shaded areas indicate “pre-tightening” periods.

regard to its intentions. This is yet another candidate explanation for the 2004
conundrum, and by including the Eurodollar futures rate, the BRS (2004) specification may be able to capture this effect. As shown in Figure 3, in the 1977, 1983,
1987, and 1994 tightening cycles, there is virtually no tendency for the two-yearyield to anticipate the increase in the federal funds rate; only once the tightening
commences does the yield rise.
Figure 4 shows that the 2004–05 episode was, in fact, different: the two-year yield
began rising three months before the first increase in the federal funds rate, and was

124 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

Comment

Figure 3 The Two-Year Yield during Tightening Periods, 1977–94

3

Percent
1977
1983
1987
1994
Average

2

1

0

–1
(12)

(10)

(8)

(6)

(4)

(2)

0

2
4
6
8
Months (before) after first rate hike

Note: Shaded area indicates the month of the first rate hike.

Figure 4 The Two-Year Yield during Tightening Periods, 1999 and 2004

3

Percent
1999
2004
Pre-1999 average

2

1

0

–1
(12)

(10)

(8)

(6)

(4)

(2)

0

2
4
6
8
Months (before) after first rate hike

Note: Shaded area indicates the month of the first rate hike.

a full percentage point higher when the tightening began. The problem, from thestandpoint of explaining the conundrum, is that a similar pattern was observed before
the 1999 tightening cycle, which was also apparently anticipated several months
prior to the first rate hike. An increase in transparency therefore cannot explain boththe near-zero excess returns in 2004 and the negative excess returns in 1999.

125

To see whether the negative pre-tightening returns might reflect a forecastable risk
premium, the excess return was regressed on two of the most likely suspects: theforward rate factor identified by Cochrane and Piazzesi (2005), and the real activity
factor constructed by Ludvigson and Ng (2005).29 Both have been shown to haveconsiderable forecasting power—the Cochrane-Piazzesi factor alone typically yields
R-squareds of more than 0.40 in excess return regressions.
Neither can explain the prevalence of negative excess returns during the periods
prior to Fed tightenings, however. While the Cochrane-Piazzesi factor performs welloverall in forecasting the excess return on 10-year Treasuries, it does not generally predictnegative returns in these pre-tightening episodes. And while the Ludvigson-Ng real
activity does a good job of predicting excess returns at the short end of the maturity
spectrum, it has relatively little forecasting power at the longer, 10-year maturity.

V. Conclusions
In the end, this cursory analysis, like that in the authors’ paper, confirms the conundrum: long-term yields during the 2004–05 episode really did behave differently
than they did in the past. The paper also provides an excellent example of how stateof-the-art macro-finance models might be used to address such questions, rigorously
integrating finance theory with sensible macroeconomic models. That the analysis
failed to uncover the proverbial “smoking gun” is in no way a shortcoming of thetechnique—although the analysis did reveal some questionable aspects of the affine
term structure method.
Although the paper meticulously covered a number of interesting hypotheses, two
did not receive the attention they deserved. One is that there has been a change in the
expectations formation process, resulting in inflation expectations that are better“anchored” than in the past. The second is a reduction in the equilibrium real interest
rate resulting from factors exogenous to the United States, such as a global savings
glut. Perhaps future research will show that one of these—or some combination of thetwo—is capable of resolving Greenspan’s conundrum.

ReferencesAng, Andrew, and Monika Piazzesi, “A No-Arbitrage Vector Autoregression of Term Structure Dynamics
with Macroeconomic and Latent Variables,” Journal of Monetary Economics, 50 (4), 2003,
pp. 745–787.
Bernanke, Ben S., Vincent R. Reinhart, and Brian P. Sack, “Monetary Policy Alternatives at the ZeroBound: An Empirical Assessment,” Brookings Papers on Economic Activity, 2, 2004, pp. 1–78.
Cochrane, John, and Monika Piazzesi, “Bond Risk Premia,” American Economic Review, 95 (1), 2005,
pp. 138–160.

29. To forecast excess returns on two- through five-year securities, Cochrane and Piazzesi (2005) constructed a factor
from the one-year rate and the two- through five-year-ahead forward rates. To forecast the excess return on a
10-year security, a procedure analogous to theirs was used to construct a forward rate factor from the one-year
rate, and the two- through nine-year-ahead forward rates.

126 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006

General Discussion

Flavin, Marjorie A., “The Adjustment of Consumption to Changing Expectations about Future
Income,” Journal of Political Economy, 89 (5), 1981, pp. 974–1009.
Gürkaynak, Refet, Brian P. Sack, and Eric T. Swanson, “The Sensitivity of Long-Term Interest Rates to
Economic News: Evidence and Implications for Macroeconomic Models,” American Economic
Review, 95 (1), 2005, pp. 425–436.
Hall, Robert E., “Stochastic Implications of the Life Cycle-Permanent Income Hypothesis: Theory andEvidence,” Journal of Political Economy, 86 (6), 1978, pp. 971–987.
Laubach, Thomas, and John Williams, “Measuring the Natural Rate of Interest,” Review of Economics
and Statistics, 85 (4), 2003, pp. 1063–1070.
Ludvigson, Sydney, and Serena Ng, “Macro Factors in Bond Risk Premia,” NBER Working Paper
No. 11703, National Bureau of Economic Research, 2005.
Rudebusch, Glenn D., Eric T. Swanson, and Tao Wu, “The Bond Yield ‘Conundrum’ from a MacroFinance Perspective,” Monetary and Economic Studies, 24 (S-1), Institute for Monetary andEconomic Studies, Bank of Japan, 2006, pp. 83–109 (this issue).
———, and Tao Wu, “A Macro-Finance Model of the Term Structure, Monetary Policy, and theEconomy,” working paper, Federal Reserve Bank of San Francisco, 2004.
Svensson, Lars E. O., “Estimating and Interpreting Forward Interest Rates: Sweden 1992–1994,”
Sveriges Riksbank Quarterly Review, 3, 1995, pp. 13–26.

General Discussion
As a rejoinder to the discussants, Glenn D. Rudebusch responded to the commentson specifications and results of the models. In response to Carsten Detken’s commenton the model specifications and a latent slope factor in the Rudebusch-Wu model, hestated that the model was parsimonious and the latent slope factor worked remarkably well in terms of Taylor rule monetary policy interpretation. Regarding theinconsistency of the idea to explain the ex post residual on other variables, Rudebusch
acknowledged that they could have been added to the models but did not come out
successfully. In response to Kenneth N. Kuttner’s question on how to choose thedependence of risk price variability on certain factors, Rudebusch explained thathe chose it arbitrarily and there was a whole set of variables that could affect therisk price. Regarding the difficulties in estimation due to nonlinearity of coefficients
relating yields to the state variables, Rudebusch acknowledged them as a drawback
to these models.
Regarding the distinction between the models that yield different decomposition
results into the risk-neutral yield and term premium, Rudebusch’s co-author, Tao Wu(Federal Reserve Bank of Dallas), explained that the expectations of the future
short-term rate would revert to some time-varying value in the Rudebusch-Wu
model. He added that this was contrary to the Bernanke-Reinhart-Sack (BRS) model,
where it would approach its sample mean very quickly. He stated that this was thereason why the estimates of the risk-neutral 10-year yield in the Rudebusch-Wu
model were more volatile than in the BRS model. Rudebusch expressed his view thata drifting process better approximated inflation over the estimation period than amean-reverting process.
In the general discussions, there were some comments on candidate explanationsof the conundrum examined in the paper. Hung Tran (International Monetary Fund)claimed that the demand by pension funds for which the authors could not find a127

proxy was worth exploring because a variety of regulatory and accounting changes in
the United States and Europe would keep the demand for long-term government
bonds high. Rudebusch responded that if the conundrum had gone away in the last
six months, pension fund reform would not explain the conundrum. Seng-Guan Toh(Bank Negara Malaysia) asked for a quantitative assessment of the contribution of aprecommitment or the change in communication by the Federal Reserve to explaining the conundrum. Rudebusch responded that he tried to account for transparency
at the Fed in terms of the implied volatility of the short-term Eurodollar options but
it did not work. Thammarak Moenjak (Bank of Thailand) proposed to take a firstdifference of some variables used in regressions because they were not significant due
to shifting means.
As alternative explanations for the conundrum, many participants stressed theimportance of global factors. Masaaki Shirakawa (Bank of Japan) pointed out that if
there was a conundrum in the U.S. bond yields, Japanese government bonds would be
attractive and thus the conundrum would be exported from one country to another.
Emphasizing the increased correlation between U.S. and Japanese bond yields, heproposed that the global factor be incorporated into the analysis. Kenneth J. Singleton(Stanford University) pointed out that a culprit for the conundrum might be theextensive degree to which carry trades are being taken on around the world. Having
looked at the very similar conundrum in Europe that Detken showed, Fabrizio Perri(New York University) commented that a global explanation was probably in order.
He also pointed out that high oil prices would help to explain the conundrum because
one possible explanation of a world savings glut was high oil prices that increase thewealth of some oil-producing countries and these countries preferred to invest in euro
and U.S. bonds. David Longworth (Bank of Canada) remarked that then-Chairman
Greenspan’s statement of the conundrum was not about the United States but about
the world and therefore some of explanations that were put forward could not be
assessed, unless we had a model in the context of the world.
Rudebusch agreed that the conundrum might be an international phenomenon.
In response to Singleton, he stated that the carry trade was probably closely related tothe conundrum but he had not thought about how to model it or structure a discussion.
He agreed with Perri that high oil prices would be a manifestation of the global savings
glut. He also pointed out that examining excess savings not just from oil-producing
countries but also from Asia would be one way to think about the conundrum.
Some questions were raised on interpretations of the conundrum. Ulrich Kohli(Swiss National Bank) argued that if monetary policy was tightened more than
expected, one would end up with lower inflation and lower nominal interest rates in
the future and therefore it would not be surprising to see the long rates going downwhen interest rates were rising at the short end. Rudebusch responded that the rise inlong rates with imperfect credibility was a little more typical than the decrease withperfect credibility. Shirakawa questioned the distinction between a conundrum and a
bubble. Rudebusch responded that the term “bond market bubble” was not normallyused, because a rational bubble with finite maturity was impossible whereas some
irrational bubble is possible.

128 MONETARY AND ECONOMIC STUDIES (SPECIAL EDITION)/DECEMBER 2006