The acceleration of the U.S. productivity growth in the late 1990s suggests a significant advance in technological innovation, making the perceived probability of entering a “new economy” ever increasing. Based on macroeconomic data, we identify a Bayesian investor's belief evolution when facing a possible structural break in the economy. We show that such belief evolution plays a significant role in explaining both the stock market boom and crash during 1998 to 2001. We conclude that a rational investor's uncertainty about the future of the U.S. economy provides an alternative explanation for the late 1990s stock market “bubble.”

The acceleration of the U.S. productivity growth in the late 1990s suggests a significant advance in technological innovation, making the perceived probability of entering a “new economy” ever increasing. Based on macroeconomic data, we identify a Bayesian investor's belief evolution when facing a possible structural break in the economy. We show that such belief evolution plays a significant role in explaining both the stock market boom and crash during 1998 to 2001. We conclude that a rational investor's uncertainty about the future of the U.S. economy provides an alternative explanation for the late 1990s stock market “bubble.”

Agency mortgage‐backed securities (MBSs) with diverse characteristics are traded in parallel through individualized specified pool (SP) contracts and standardized to‐be‐announced (TBA) contracts with delivery flexibility. This parallel trading environment generates distinctive effects on MBS pricing and trading: (i) Although cheapest‐to‐deliver (CTD) issues are present in TBA trading and absent from SP trading by design, MBS heterogeneity associated with CTD discounts affects SP yields positively, with the effect stronger for lower‐value SPs; (ii) high selling pressure amplifies the effects of MBS heterogeneity on SP yields; and (iii) greater MBS heterogeneity dampens SP and TBA trading activities but increases their ratio.