Bubble Necessity Theorem

Asset price bubbles are situations where asset prices exceed the fundamental values defined by the present value of dividends. This paper presents a conceptually new perspective: the necessity of bubbles. We establish the bubble necessity theorem in a plausible general class of economic models: with faster long-run economic growth (G) than dividend growth (Gd) and counterfactual long-run autarky interest rate (R) below dividend growth, all equilibria are bubbly with nonnegligible bubble sizes relative to the economy. This bubble necessity condition naturally arises in economies with sufficiently strong savings motives and multiple factors or sectors with uneven productivity growth.