Abstract
We study in a New Keynesian framework the consequences of adaptive learning for the design of robust monetary policy. Compared to rational expectations, the fact that private sector follows adaptive learning gives the central bank an additional intertemporal trade-off between optimal behavior in the present and in later periods thanks to its ability to manipulate future inflation expectations. We show that adaptive learning imposes a more restrictive constraint on monetary policy robustness to ensure the dynamic stability of the equilibrium than under rational expectations but strengthens the argument in favor of a more aggressive monetary policy when the central bank fears for model misspecifications.
Acknowledgement
We are grateful to Jean-Bernard Chatelain, Francesco De Palma, Amélie Barbier-Gauchard, Rodolphe Dos Santos Ferreira, Moïse Sidiropoulos, Marco Maria Sorge and an anonymous reviewer for helpful remarks and suggestions.
A Appendix
In subsections A.1 and A.2, we closely follow Molnár and Santoro (2014) to find the equilibrium solution under learning. In subsections A.3 and A.4, we develop original techniques to show the effects of learning and robustness on the equilibrium.
A.1 The equilibrium solution of inflation under learning
Substituting
Using
Substituting xt and xt+1 given by (33)–(34) into (32) and arranging the terms yields:
with
According to the proposition 1 from Blanchard and Kahn (1980), the ALM solution for inflation takes the following form :
Advancing (39) one period and taking the expectation of the resulting equation while using (6) yield:
Using (35) to eliminate
Comparing (39) and (41) yields:
and
We gather equations (6), (7) and (35), while using (33) to substitute xt to obtain the system of three equations:
where
The above system is subject to three boundary conditions: a0, b0, and
We can show that, in Appendix A.2, A1 has an eigenvalue inside and one outside the unit circle.□
A.2 The single stable solution
Among infinite stochastic sequences satisfying equation (42), we focus on a non-explosive solution, i.e. the solution corresponding to the eigenvalue of A1 given by (44) inside the unit circle. The trace and determinant of A1 are both positive. Thus, for A1 to have two real eigenvalues (μ1, μ2), one inside and one outside the unit circle, it is sufficient to show that
Knowing that
After simplification, we get:
which is verified given that
There exists a unique solution to the model, whose ALM takes the following form:
To have a converging (and non-explosive) inflation, we must have
where
To characterize the two solutions of
We rewrite p1, after some tedious calculus, as
or alternatively simplify it as
The conditions imposed on θ to ensure that
Under RE, to ensure the dynamic stability of the equilibrium, we must have according to (9) that
For
To ensure that
The stability condition given by (51) is too loose compared to condition (27), i.e.
The stable solution of
The other possible solution
We now show that
Using
Given that
The case whereγ
It follows from (42)–(43) that
The case whereγ
Substituting the latter into (36)–(38) leads to
A.3 The effects of learning
Deriving p0, p1 and p2 with respect to γ and using (50), we get:
Deriving
which can be rewritten, using
Using
It is easy to check that for γ = 1, we have
if
Deriving H with respect to γ yields
Consequently, given that H < 0 for γ = 1 and
Deriving
where
It follows that
Using the definition of
A.4 Effects of robustness
Deriving p0, p1 and p2 with respect to θ and using (50), we get:
Deriving
where
Using
Deriving
Deriving
To ensure that
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Article note
A technical appendix is available upon request to the author.
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