We study the effectiveness of public investment in stimulating an economy stuck in a liquidity trap. We do so in the context of a tractable new-Keynesian economy in which a fraction of government spending increases the stock of public capital subject to a time-to-build constraint. Public investment projects typically entail significant time-to-build delays, which often span several years from approval to completion. We show that this feature implies that the spending multiplier associated with public investment can be substantially large-nearly twice as large as the multiplier associated with public consumption-in a liquidity trap. Intuitively, when the time to build is sufficiently long, and to the extent that public capital raises the marginal productivity of private inputs, the resulting disinflationary effect will occur after the economy has escaped from the liquidity trap. At the same time, the increase in households' expected wealth amplifies aggregate demand while the economy is still in the liquidity trap. Using a mediumscale model extended to allow for the accumulation of public capital, we quantify the multiplier associated with the spending component of the 2009's ARRA, which allocated roughly 40% of the authorized funds to public investment. We find a peak multiplier of 2.31. Our results also indicate that failing to account for the composition of the stimulus by overlooking its investment component would lead one to underestimate the spending multiplier by about 50%.

We study optimal monetary and fiscal policy under commitment in an economy where monetary policy is constrained by the zero lower bound on the nominal interest rate, and where the government can allocate spending to public consumption and public investment. We show that the optimal response to an adverse shock that precipitates the economy into a liquidity trap entails a small and short-lived increase in public consumption but a large and persistent increase in public investment, which lasts well after the natural rate of interest has ceased to be negative. During this period, the optimal composition of public spending is therefore heavily skewed towards public investment. Contrary to the literature that abstracts from public investment, we find that the optimal increase in total public spending in a deep recession is sizable. However, we show that this fiscal expansion has little to do with a stabilization motive and is instead warranted by the intertemporal allocation of resources that efficiency dictates even in the absence of an output gap.

We investigate the size of the multiplier at the ZLB in a New keynesian model. It ranges from around -0.25 to +1.5, depending on the extent to which government spending is productive, substitutable or not for private consumption.

Abstract:Public investment represents a non-negligible fraction of total public expenditures. Yet, theoretical studies of the effects of public spending when the economy is stuck in a liquidity trap invariably assume that government expenditures are entirely wasteful. In this paper, we consider a new-Keynesian economy in which a fraction of government spending increases the stock of public capital-which is an external input in the production technology-subject to a time-to-build constraint. In this environment, an increase in public spending has two conflicting effects on current and expected inflation: a positive effect due to higher aggregate demand and a negative effect reflecting future declines in real marginal cost. We solve the model analytically both in normal times and when the zero lower bound (ZLB) on nominal interest rates binds. We show that under relatively short time-to-build delays, the spending multiplier at the ZLB decreases with the fraction of public investment in a stimulus plan. Conversely, when several quarters are required to build new public capital, this relationship is reversed. In the limiting case where a fiscal stimulus is entirely allocated to investment in public infrastructure, the spending multiplier at the ZLB is 4 to 5 times larger than in normal times when the time to build is 12 quarters.