美國90年代經濟增長的新解釋(PDF 52頁)

ABSTRACT
We propose a framework for understanding recurrent historical episodes of vigorous economic
expansion accompanied by extreme asset valuations, as exhibited by the U.S. in the 1990s. We
interpret this phenomenon as a high-valuation equilibrium with a low effective cost of capital based
on optimism about the future availability of funds for investment. The key to the sustainability of
such an equilibrium is feedback from increased growth to an increase in the supply of effective
funding. We show that such feedback arises naturally when an expansion comes with technological
progress in the capital producing sector, when fiscal rules generate sustained fiscal surpluses, when
the rest of the world has lower expansion potential, and when financial constraints are relaxed by
the expansion itself. Arguably, these ingredients were all simultaneously present in the U.S. during
the 1990s. We also show that such expansions can be welfare improving but they can crash. The
latter is more likely if bubbles develop along the expansionary path. These (rational) bubbles can
emerge even when the interest rate exceeds the rate of growth of the economy.
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