Competition among health maintenance organizations

We develop a model of competition among health maintenance organizations (HMOs) to analyze the effects of market power, scale economies, and asymmetric knowledge of health risk on market outcomes. We find that competition among HMOs may, but need not, ensure socially preferred outcomes. Market power or scale economies can sometimes admit socially preferred outcomes when they would otherwise not arise. Asymmetric knowledge of health risk may or may not be constraining. When it is constraining, a variety of patterns of incomplete health insurance can arise, along with excessive or insufficient treatment and preventive care for either high-risk or low-risk individuals.