The law's willingness to take account of factors that interfere with volition tends to vary in accordance with its underlying goals. The law of wills is dominated by the principle of freedom of testation; it has thus developed doctrines aimed at detecting coercive influences that interfere with the testator's free agency. The law of medical decision making, dominated by the analogous principle of patient autonomy, has not developed doctrines aimed at detecting coercive influences despite a large and growing body of evidence showing that disordered insight and major depression, two common medical conditions, often have a coercive, negative effect on treatment choice and compliance. When a patient is afflicted with disordered insight or major depression, a decision against treatment often stems from illness instead of the patient's own goals and values. Current law fails to protect vulnerable patients whose free agency has been lost to their illnesses. These patients need, and deserve, protection from the coercive effects of distorted perception and motivation. The undue-influence and insane-delusion doctrines developed within the law of wills to detect and disarm coercive influences are readily adaptable to the medical decision-making context. There is a wealth of assessment protocols that offer reliable methods of detecting the influence of depression and insight deficiencies.