Abstract Regulations to design private yet publicly sponsored health insurance markets are increasingly adopted in many OECD countries. Here I combine data and economic theory to analyse the interaction between insurers’ competition and the design of premium subsidies in determining equilibrium outcomes. My empirical model includes adverse selection, rich heterogeneity in preferences for vertically and horizontally differentiated plans and accommodates alternative assumptions on pricing conduct. In the context of the Affordable Care Act in the U.S., I estimate the joint distribution of preferences and expected cost using Californian administrative records on 3.4 million plan choices between 2014 and 2017, combined with plan and survey data on medical claims. An empirical horse race between conduct assumptions favours oligopoly pricing over perfect competition. Considering alternative subsidy designs shows that, in equilibrium, shifting subsidy generosity toward the “young invincibles” would lower premiums for all enrolees while increasing enrolment and profits.

Under the Affordable Care Act, individual states have discretion in how they define coverage regions, within which insurers must charge the same premium to buyers of the same age, family structure, and smoking status. We exploit variation in these definitions to investigate whether the size of the coverage region affects outcomes in the ACA marketplaces. We find large consequences for small and rural markets. When states combine small counties with neighboring urban areas into a single region, the included rural markets see 0.6 to 0.8 more active insurers, on average, and savings in annual premiums of between $200 and $300.