By steering patients to cost-effective substitutes, the tiered design of prescription drug formularies can improve the efficiency of healthcare consumption in the presence of moral hazard. However, a long theoretical literature describes how contract design can also be used to screen consumers by profitability. In this paper, we study this type of screening in the ACA Health Insurance Exchanges. We first show that despite large regulatory transfers that neutralize selection incentives for most consumer types, some consumers are unprofitable in a way that is predictable by their prescription drug demand. Then, using a difference-in-differences strategy that compares Exchange formularies where these selection incentives exist to employer plan formularies where they do not, we show that Exchange insurers design formularies as screening devices that are differentially unattractive to unprofitable consumer types. This results in inefficiently low levels of coverage for the corresponding drugs in equilibrium. Although this type of contract distortion has been highlighted in the prior theoretical literature, until now empirical evidence has been rare. The impact on out-of-pocket costs for consumers affected by the distortion is substantial--potentially thousands of dollars per year--and the distortion creates an equilibrium in which contracts that efficiently trade off moral hazard and risk protection cannot exist.
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