Legal institutions that index legal capacity to decision-making capacity are often inexplicit on the theoretical basis for holding that an individual—minor or adult—possesses decision-making capacity. I argue that a theoretical reconstruction of decision-making capacity offers illumination to the law. I have two objectives. First, to establish that the relevant doctrines of English law—the Mental Capacity 2005, s 3(1) and Gillick competence tests—are best explained by rational choice theory. That is, best understood as evaluating the instrumental rationality of an individual’s decision—whether an individual’s choice(s) in respect of a matter at a time accord with what she has most reason to do, in light of what she believes about the world and what she values in the world. I argue that the case for associating functional decision-making capacity and instrumental rationality is easily made out in respect of the MCA 2005 and is plausible in respect of the Gillick test. Second, having made the case for the association of functional decision-making capacity and instrumental rationality, I show that rational choice theory illuminates vexed matters of English legal doctrine in respect of minor decision-making capacity and medical treatment: 1) whether capacity is ‘risk-relative’, that is, whether an individual’s capacity to consent is asymmetric to her capacity to refuse or vice versa; 2) the doctrine that there exists a distinction between capacity to refuse consent to one treatment and capacity to refuse all treatment. I show that the risk relative standard of capacity is mistaken in light of the importance of subjective values to rational choice theory and how agents form preference relations within the theory. I show that the distinction between capacity to refuse consent and capacity to refuse treatment relies on whether we count available or possible interventions among an individual’s options and how we treat multiple options.