Rapid advances in neuroscience may enable us to identify the neural correlates of ordinary decision making. Such knowledge opens up the possibility of acquiring highly accurate information about people's competence to consent to medical procedures and to participate in medical research. Currently we are unable to determine competence to consent with accuracy and we make a number of unrealistic practical assumptions to deal with our ignorance. Here I argue that if we are able to detect competence to consent and if we are able to develop a reliable neural test of competence to consent, then these assumptions will have to be rejected. I also consider and reject three lines of argument that might be developed by a defender of the status quo in order to protect our current practices regarding judgments of competence in the face of the availability of information about the neural correlates of ordinary human decision making.