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In recent years, measured TFP growth in the US has declined. We argue that two forces contributed to this decline: the mismeasurement of intangible capital, and rising markups. Markups affect input shares, while intangibles omitted from measures of investment affect measured capital growth, each potentially generating downward bias in measured TFP growth. Most importantly, when both forces are simultaneously present, their effects reinforce each other and amplify the downward bias in measured TFP growth. Using input-output data, we estimate that this mechanism could account for one-third to two-thirds of the decline in measured TFP growth.

The best predictor of current investment at the firm level is lagged investment. This lagged-investment effect is empirically more important than the cash-flow and Q effects combined. We show that the specification of investment adjustment costs proposed by Christiano et al. (2005) predicts the presence of a lagged-investment effect and that a generalized version of their model is consistent with the behavior of firm-level data from Compustat.

When investment decisions cannot be reversed and returns to capital are uncertain, the firm faces a higher user cost of capital than if it could reverse its decisions. This higher user cost tends to reduce the firm's capital stock. Opposing this effect is the irreversibility constraint itself: when the constraint binds, the firm would like to sell capital but cannot. This effect tends to increase the firm's capital stock. We show that a firm with irreversible investment may have a higher or a lower expected capital stock, even in the long run, compared to an otherwise identical firm with reversible investment. Furthermore, an increase in uncertainty can either increase or decrease the expected long-run capital stock under irreversibility relative to that under reversibility. However, changes in the expected growth rate of demand, the interest rate, the capital share in output, and the price elasticity of demand all have unambiguous effects.