What Drives Money Competition: Comparative Advantage in Payments versus Reserves

We study competition between monies that provide separate payment and non-payment (e.g., storeof-value) functions.Our central insight is that payment adoption is governed not by absolute payment superiority, but by comparative advantage between payment and non-payment roles.A money that is "too good" as a store of value may circulate less as a payment instrument, even if it is technologically superior, because agents prefer to hoard it rather than spend it.The model delivers equilibria in which monies either specialize into distinct roles or coexist as payment instruments with one emerging as dominant.These mechanisms provide a unified microfoundation for classic monetary phenomena such as Gresham's law and the big problem of small change, and offer a new perspective on modern debates over stablecoins and central bank digital currencies (CBDCs).Contrary to the common view that interest-bearing digital currencies necessarily threaten bank deposits, we show that higher yields can weaken payment adoption by raising the opportunity cost of spending.As a result, traditional bank deposits may coexist with, and even retain dominance over, technologically superior digital alternatives.