When do nudges undermine voluntary consent?

The permissibility of nudging in public policy is often assessed in terms of the conditions of transparency, rationality, and easy resistibility. This debate has produced important resources for any ethical inquiry into nudging, but it has also failed to focus sufficiently on a different yet very important question, namely: when do nudges undermine a patient's voluntary consent to a medical procedure? In this paper, I take on this further question and, more precisely, I ask to which extent the three conditions of transparency, rationality, and easy resistibility can be applied to the assessment of voluntary consent too. After presenting two examples, designed to put pressure on these three conditions, I show that, suitably modified, the three conditions can remain significant in the assessment of voluntary consent as well. However, the needed modifications are very substantial and result in a rather complicated view. To propose a tidier solution, I argue that nudging undermines voluntary consent if and only if it cannot be 'interpersonally justified' to the patient. I use the three modified conditions to motivate the idea of interpersonal justification and also to further specify the principles it involves. My resulting view is especially attractive because it builds on already existing insights from the debate on nudging, updates those insights with an eye to medical consent, and finally unites them in an elegant and simple framework.