Supplemental insurance: Medicare's accidental stepchild.

The majority of Medicare beneficiaries supplement the basic Medicare benefit package with additional insurance. This article reviews the literature on Medicare supplemental insurance. Supplemental insurance plays a significant role in protecting Medicare beneficiaries from financial risk. The two major sources of coverage for beneficiaries--former employers and individual purchase--differ in benefit structure and characteristics of policy holders. Employer-sponsored policies tend to provide broader coverage with more cost sharing than individually purchased policies, and holders of employer policies tend to be younger, wealthier, healthier, and better educated. Supplemental insurance policies have been shown to be associated with higher Medicare expenditures, but there is no consensus on the cause of the higher expenditures. Some studies attribute the increase to adverse selection of policies; other studies point to the moral hazard effect of insurance.