With advances in medical science, the concept of agency has received increasing attention in biomedical ethics. However, most of the ethical discussion around definitions of agency has focused either on patients suffering from mental disorders or on patients receiving cutting-edge medical treatments in developed countries. Very little of the discussion around concepts of agency has focused on the situation of patients suffering from common diseases that affect populations worldwide. Therefore, the most widely-used definitions of agency may be not appropriate to (...) analyse common diseases among large populations. The branch of social sciences known as development studies draw on their own definitions of the term agency that may provide a more applicable and accurate way of referring to common and general cases than the definitions currently used in bioethics. Moreover, the psychological Self-Determination Theory may improve the usefulness of these definitions in common situations. This article explains the characteristics and the shortcomings of current bioethical definitions of agency when they are applied to common medical conditions worldwide. A new, value-based concept of agency, informed by development studies, is proposed as more accurate and useful for biomedical ethics. (shrink)