Competition in Two-Sided Markets

There are many examples of markets involving two groups of agents who need to interact via platforms, and where one groups bene\u85t from joining a platform depends on the number of agents from the other group who join the same platform. This paper presents theoretical models for three variants of such markets: a monopoly platform; a model of competing platforms where each agent must choose to join a single platform; and a model of competing bottlenecks, where one group wishes to join all platforms. The main determinants of equilibrium prices are (i) the relative sizes of the cross-group externalities, (ii) whether fees are levied on a lump-sum or per-transaction basis, and (iii) whether a group joins just one platform or joins all platforms.