Origins and definition
Keynes (1936) had already observed that stock market valuations affect the incentive to undertake new investment. Brainard and Tobin (1968) and Tobin (1969) formalized this as a ratio, q, within a general equilibrium model of financial markets. q is the market value of a firm's capital divided by the cost of replacing it. When q exceeds one, the market values newly installed capital above its cost, giving firms an incentive to issue shares or raise funds and expand investment. When q is below one, buying existing assets is cheaper than building new ones, and investment slows. Because monetary policy changes asset prices and thus q, which then feeds into investment, the theory is also treated as one channel of monetary transmission.
Marginal q and average q
What matters for investment decisions is marginal q, the value of one additional unit of capital, but what can be observed is average q, total market value divided by total replacement cost. Building on adjustment-cost models such as Lucas and Prescott (1971), Hayashi (1982) showed that marginal and average q coincide when the firm is a price taker and its production and adjustment-cost functions are homogeneous of degree one. This result allowed Summers (1981) and others to estimate empirically how taxation affects investment through q.
Empirical work and debate
q is widely used as a measure of firm value in finance and industrial organization. Lindenberg and Ross (1981) used q as a gauge of market power, Morck, Shleifer and Vishny (1988) examined how managerial ownership relates to q, and Chung and Pruitt (1994) proposed an easy-to-compute approximation. In practice, however, the link between investment and q has proved weaker than predicted. Fazzari, Hubbard and Petersen (1988) emphasized financing constraints, noting that internal cash flow explains investment beyond q. Erickson and Whited (2000) attributed much of the weak relation to measurement error in q, and Philippon (2009) reported that a q estimated from bond prices explains investment better. Peters and Taylor (2017) showed that including intangible capital such as R&D; and brands in replacement cost raises q's explanatory power. Together with the efficient market hypothesis and the Modigliani–Miller theorem, the theory is a core framework linking financial markets and real investment.
FAQ
- What is Tobin's q in simple terms?
- It compares what the market says a firm is worth with the cost of rebuilding its assets from scratch. Above one, investment tends to rise; below one, it tends to fall.
- How do marginal q and average q differ?
- Marginal q is the value of adding one more unit of capital, while average q is total market value over total replacement cost. Hayashi (1982) showed they are equal under certain conditions.
Sources reviewed: 2026-10-26
References
- Keynes, J. M. (1936). The General Theory of Employment, Interest and Money. London: Macmillan.
- Tobin, J. (1969). A general equilibrium approach to monetary theory. Journal of Money, Credit and Banking, 1(1), 15–29.
- Brainard, W. C., & Tobin, J. (1968). Pitfalls in financial model building. American Economic Review, 58(2), 99–122.
- Lucas, R. E., & Prescott, E. C. (1971). Investment under uncertainty. Econometrica, 39(5), 659–681.
- Hayashi, F. (1982). Tobin's marginal q and average q: A neoclassical interpretation. Econometrica, 50(1), 213–224.
- Summers, L. H. (1981). Taxation and corporate investment: A q-theory approach. Brookings Papers on Economic Activity, 1981(1), 67–140.
- Lindenberg, E. B., & Ross, S. A. (1981). Tobin's q ratio and industrial organization. Journal of Business, 54(1), 1–32.
- Fazzari, S. M., Hubbard, R. G., & Petersen, B. C. (1988). Financing constraints and corporate investment. Brookings Papers on Economic Activity, 1988(1), 141–206.
- Morck, R., Shleifer, A., & Vishny, R. W. (1988). Management ownership and market valuation: An empirical analysis. Journal of Financial Economics, 20, 293–315.
- Chung, K. H., & Pruitt, S. W. (1994). A simple approximation of Tobin's q. Financial Management, 23(3), 70–74.
- Erickson, T., & Whited, T. M. (2000). Measurement error and the relationship between investment and q. Journal of Political Economy, 108(5), 1027–1057.
- Philippon, T. (2009). The bond market's q. Quarterly Journal of Economics, 124(3), 1011–1056.
- Peters, R. H., & Taylor, L. A. (2017). Intangible capital and the investment-q relation. Journal of Financial Economics, 123(2), 251–272.