We test whether comovements of individual stock prices can be justified by economic fundamentals. This is a test of the present value model of security valuation with the constraint that changes in discount rates depend only on changes in macroeconomic variables. Then, stock prices of companies in unrelated lines of business should move together only in response to changes in current or expected future macroeconomic conditions. Using a latent variable model to capture unobserved expectations, we find excess comovement of returns. We show that this excess comovement can be explained in part by company size and degree of institutional ownership, suggesting market segmentation.

Capital investment decisions must recognize the limitations on the firm's ability to later sell or expand capacity. This paper shows how opportunities for future expansion or contraction can be valued as options, how their valuation relates to the q theory of investment, and their effect on the incentive to invest. Generally, the option to expand reduces the incentive to invest, while the option to disinvest raises it. We show how these options determine the effect of uncertainty on investment, how they are changed by shifts of the distribution of future profitability, and how the q-theory and option pricing approaches are related.