Gross domestic product growth rates as confined Lévy flights: Towards a unifying theory of economic growth rate fluctuations.

A model that combines economic growth rate fluctuations at the microscopic and macroscopic levels is presented. At the microscopic level, firms are growing at different rates while also being exposed to idiosyncratic shocks at the firm and sector levels. We describe such fluctuations as independent Lévy-stable fluctuations, varying over multiple orders of magnitude. These fluctuations are aggregated and measured at the macroscopic level in averaged economic output quantities such as GDP. A fundamental question is thereby to what extent individual firm size fluctuations can have a noticeable impact on the overall economy. We argue that this question can be answered by considering the Lévy fluctuations as embedded in a steep confining potential well, ensuring nonlinear mean-reversal behavior, without having to rely on microscopic details of the system. The steepness of the potential well directly controls the extent to which idiosyncratic shocks to firms and sectors are damped at the level of the economy. Additionally, the theory naturally accounts for business cycles, represented in terms of a bimodal economic output distribution and thus connects two so far unrelated fields in economics. By analyzing 200 years of U.S. gross domestic product growth rates, we find that the model is in good agreement with the data.