Recent theoretical research implies that the explanatory power of the size and book-to-market factors for the cross section of stock returns arises from return predictability associated with businesses’ optimal investment decisions. In the models, book-to-market ratio and size change as firms exercise investment options and their existing assets depreciate. As a result, exposure to systematic risk and hence expected stock returns evolve in a predictable manner (Berk et al. (1999), Gomes et al. (2003), Carlson et al. (2004), Cooper (2006)).