This paper introduces model uncertainty into a behavioral New Keynesian DSGE framework and derives robust optimal monetary policies. We build a heterogeneous agents DSGE model, where a fraction of agents behave according to some forms of bounded rationality (boundedly rational agents), while the reminder operate on the basis of expectations that are corrected on average (rational agents). We consider two potential mechanisms of expectations formation to generate beliefs. The central bank observes the aggregate economic dynamics, but it ignores the fraction of boundedly rational agents and/or the mechanism they use to form their expectations. Non-Bayesian robust control techniques are then adopted to minimize a welfare loss derived from the second-order approximation of agents' utilities. We account of model uncertainty considering both commitment and discretion regime.

This paper presents a framework for analyzing how bounded rationality affects monetary and fiscal policy. The model is a tractable and parsimonious enrichment of the widely-used New Keynesian model – with one main new parameter, which quantifies how poorly agents understand future policy and its impact. That myopia parameter, in turn, affects the power of monetary and fiscal policy in a microfounded general equilibrium. A number of consequences emerge. (i) Fiscal stimulus or \helicopter drops of money" are powerful and, indeed, pull the economy out of the zero lower bound. More generally, the model allows for the joint analysis of optimal monetary and fiscal policy. (ii) The Taylor principle is strongly modified: even with passive monetary policy, equilibrium is determinate, whereas the traditional rational model yields multiple equilibria, which reduce its predictive power, and generates indeterminate economies at the zero lower bound (ZLB). (iii) The ZLB is much less costly than in the traditional model. (iv) The model helps solve the “forward guidance puzzle”: the fact that in the rational model, shocks to very distant rates have a very powerful impact on today's consumption and inflation: because agents are partially myopic, this effect is muted. (v) Optimal policy changes qualitatively: the optimal commitment policy with rational agents demands “nominal GDP targeting”; this is not the case with behavioral firms, as the benefits of commitment are less strong with myopic forms. (vi) The model is “neo-Fisherian” in the long run, but Keynesian in the short run: a permanent rise in the interest rate decreases inflation in the short run but increases it in the long run. The non-standard behavioral features of the model seem warranted by the empirical evidence.

We study the effects of model uncertainty in a simple New-Keynesian model using robust control techniques. Due to the simple model structure, we are able to find closed-form solutions for the robust control problem, analysing both instrument rules and targeting rules under different timing assumptions. In all cases but one, an increased preference for robustness makes monetary policy respond more aggressively to cost shocks but leaves the response to demand shocks unchanged. As a consequence, inflation is less volatile and output is more volatile than under a non-robust policy. Under one particular timing assumption, however, increasing the preference for robustness has no effect on the optimal targeting rule (nor on the economy).

We analytically characterize optimal monetary policy for an augmented New Keynesian model with a housing sector. With rational private sector expectations about housing prices and inflation, optimal monetary policy can be characterized by a standard "target criterion" in terms of inflation and the output gap, that makes no reference to housing prices. If instead the policymaker is concerned with potential departures of private sector expectations from rational ones, and seeks a policy that is robust against such possible departures, then the optimal target criterion will also depend on housing prices. For empirically realistic cases, robustness requires the central bank to "lean against" housing prices, i.e., to adopt a stance that is projected to undershoot (overshoot) its normal targets for inflation and the output gap following unexpected housing price increases (decreases). Notably, robustly optimal policy does not require that the central bank distinguish between "fundamental" and "non-fundamental" movements in housing prices.

We use robust control to study how a central bank in an economy with imperfect interest rate pass-through conducts monetary policy if it fears that its model could be misspecified. The effects of the central bank's concern for robustness can be summarised as follows. First, depending on the shock, robust optimal monetary policy under commitment responds either more cautiously or more aggressively. Second, such robustness comes at a cost: the central bank dampens volatility in the inflation rate preemptively, but accepts higher volatility in the output gap and the loan rate. Third, if the central bank faces uncertainty only in the IS equation or the loan rate equation, the robust policy shifts its concern for stabilisation away from inflation.

This paper analyzes the use of unconventional policy instruments in New Keynesian setups in which the ‘divine coincidence’ breaks down. The paper discusses the role of a second instrument and its coordination with conventional interest rate policy, and presents theoretical results on equilibrium determinacy, the inflation bias, the stabilization bias, and the optimal central banker’s preferences when both instruments are available. We show that the use of an unconventional instrument can help reduce the zone of equilibrium indeterminacy and the volatility of the economy. However, in some circumstances, committing not to use the second instrument may be welfare improving (a result akin to Rogoff (1985a) example of counterproductive coordination). We further show that the optimal central banker should be both aggressive against inflation, and interventionist in using the unconventional policy instrument. As long as price setting depends on expectations about the future, there are gains from establishing credibility by using any instrument that affects these expectations.