 Misjudgments concerning the efficacy of therapies for purely epistemic reasons are not the only worry that one might have about expert opinion. Medical experts and patients are in what economists call a principal-agent relationship. The principal—in this case, the patient—desires the delivery of a certain good or service—in this case, his health. He instructs an agent—in this case, the doctor—with it, because he lacks the expertise to produce the good himself. The good can only be produced with uncertainty: no therapy is 100% effective. Moreover, the success at delivering the good depends in part on the agent’s effort. The doctor may not always choose the optimal therapy for a patient (we can suppose that it takes some effort to select the optimal therapy for a patient), and any therapy can be implemented sloppily. Moreover, lacking expertise, the patient cannot observe the level of effort a doctor puts in. He therefore cannot design a contract that makes payments dependent on level of effort (much less on success, as success is in part influenced by factors outside of either party’s control). Agents therefore have an incentive to cheat: not to put in the level of effort required to select and deliver the optimal therapy from a patient’s point of view.