Nudges, one of the rapidly growing law-making and public policy tools, are considered by their proponents to have a generally neutral effect on autonomy. Our study is the first to test nudges under the prism of Self-Determination Theory, a motivational theory which posits that autonomy is a basic psychological need. We focus on a specific type of nudge, defaults, and test it within the context of making a choice among a hypothetical set of insurance programs for post-graduate students.Results show that the experience of an Internal Perceived Locus of Causality (I-PLOC) is negatively affected by defaults when the number of options is low but there is no effect when it is high. In other words, people are less likely to view themselves as the origin of their choices when defaults are in place and when options are manageable. The experience of an I-PLOC has a positive effect on self-regulation and vitality, partially mediated by perceived competence. We suggest that since even mild manipulations show an undermining effect on autonomy, the so-called 'libertarian paternalistic' interventions should be evaluated on the basis of their effect on basic need satisfaction, self-regulation and vitality, all of which are associated with autonomy according to Self-Determination Theory.