Recognising that offers of payment to research participants can serve various purposes—reimbursement, compensation and incentive—helps uncover differences between participants, which can justify differential payment of participants within the same study. Participants with different study-related expenses will need different amounts of reimbursement to be restored to their preparticipation financial baseline. Differential compensation can be acceptable when some research participants commit more time or assume greater burdens than others, or if inter-site differences affect the value of compensation. Finally, it may be permissible (...) to offer differential incentive payments if necessary to advance the goals of a study. We encourage investigators and Institutional Review Boards to think about whether to offer payment, in what amounts and for what purpose, and also to consider whether differential payment can help promote the scientific and ethical goals of clinical research. (shrink)