Theory posits that when managers anticipate excess capacity, average q becomes a biased estimator of marginal q as the potential for underutilizing new capital reduces the marginal benefit of investing. After correcting for this source of measurement error, the explanatory power of Tobin's q substantially improves in time‐series and cross‐sectional regressions as well as in out‐of‐sample tests. These findings, together with a secular erosion in capacity utilization, help explain why corporate investment rates have been declining for decades despite average q increasing significantly. Our analysis indicates that economic rigidities have contributed to the persistent erosion in capacity utilization.