Abstract: This paper provides a dynamic model of the dual economy in which differences in productivity across sectors arise endogenously. Rather than relying on exogenous price distortions, duality arises because of differences between sectors in the separability of their fertility and labor decisions. The model demonstrates how a dual economy will originate, persist, and eventually disappear within a unified growth framework. It is also shown that agricultural productivity growth will exacerbate the inefficiencies of a dual economy and slow down long-run growth.