On interest rate policy and equilibrium stability under increasing returns: a note

Bullard and Mitra [Journal of Monetary Economics 49 (2002), 11051130] find that, in a New Keynesian economy without capital and under four variants of the Taylor rule, the Taylor principle is sufficient to guarantee both determinacy and E-stability of equilibrium in most cases. Xiao [Macroeconomic Dynamics 12 (2008), 2249] claims that with capital and mild increasing returns the Taylor principle cannot guarantee either determinacy or E-stability with any of the four rules. In this paper we show that in the Calvo-type sticky price models a second-order condition for profit maximization must be satisfied in firms' pricing decision problem, and we point out that the examples given in Xiao's paper to support his conclusion violate this condition. After imposing this condition, we find that increasing returns have little effect on determinacy and E-stability under two of the policy rules but significant effects under the other two. These results are obtained in models both with and without capital. © 2009 Copyright Cambridge University Press.