This paper examines the conditions under which the social opportunity cost of capital in a tax-distorted economy is equal to the gross-of-tax return to capital plus the excess of the market over the shadow wage bill. Whenever public investment serves as an unpriced (or underpriced) input in the private sector's production process, only part of the excess of the market over the shadow wage bill should be attributed to private-sector capital. In addition, whenever some of the benefits of public investment are appropriated by owners of capital, the gross-of-tax return to capital will tend to overstate the genuine marginal product of capital. The overall implication is that the social discount rate may be less than the gross-of-tax rate of return to capital in the private sector, even if public investment crowds out private investment dollar for dollar.
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