Depreciation Rules and the Relation Between Marginal and Historical Cost

The reported cost of a product frequently contains historical cost components that reflect past investments in productive capacity. We examine a setting wherein a firm makes a sequence of overlapping capacity investments. Earlier research has identified particular accrual accounting (depreciation) rules with the property that, on a per unit basis, the historical cost of a product captures precisely its marginal cost. Relative to this benchmark, we investigate and characterize the direction and magnitude of the bias in reported historical cost that results from alternative depreciation rules, including in particular straight-line depreciation in conjunction with partial direct expensing. In addition, we demonstrate that for a reasonable range of parameter specifications the resulting bias is rather small. Finally, we apply our framework to two specific settings. First, in a regulatory context, we establish the extent to which the accounting profit margin is indicative of a firm’s pricing power in the product market. Second, we model an internal control scenario in which a manager’s performance is evaluated using residual income, and identify the distortions in investment levels that result from the use of alternative depreciation rules.