The recent surge in the price of Bitcoin has reignited debates about a cryptocurrency bubble, many of them based on intuition. This column takes a formal approach to the beliefs underlying both sides of the debate by introducing a model that builds on the classical framework for rational bubbles. If the price of a cryptocurrency can be justified from transactional demand, it may be a sound investment. However, if future investment inflows are needed to explain the price, that indicates a bubble, with payoffs resembling a Ponzi scheme.
