Classical analysis of health insurance markets often focuses on adverse selection, which creates a direct externality between the enrollees of the same health plan: under an imperfect risk adjustment, the higher the risks of my co-enrollees, the higher my cost of insurance. This has led to the view that restricting the diversity of accessible physicians may be good for policyholders, in a context where competition between health plans can lead to a death spiral for the less restrictive plan. This paper defends the opposite view that diversity might pay, because of the indirect externality between policyholders and physicians. By attracting higher risks, the less restrictive plan may also guarantee a higher level of activity to its physicians, and therefore negotiate with them a lower fee-for-service rate. By explicitly modeling the two sides of the market for health (policyholders and physicians), we are able to find examples in which competition between health plans gives a higher profit to the less restrictive plan.
