Nudges are a widely used policy tool to modify behaviour through low-cost and choice-preserving interventions to help people make choices in their best interests. Currently, however, there is limited empirical evidence on when nudges work best and with whom. To address this gap, this thesis studies moderating factors of nudgeability (i.e., susceptibility to nudges). Specifically, I investigate the impact of locus of control, cognitive ability and perceived financial scarcity on how susceptible people are to being nudged. Locus of control turns out not to be a significant predictor of nudgeability. People with low cognitive ability, and in conditions of high perceived financial scarcity are, though, in general more nudgeable. But this effect turns out to be crucially moderated by whether a nudge is helpful (i.e., in the best interests of the person being nudged) or harmful (i.e., acting against their interests). The key finding is that harmful nudges are especially influential for people with low cognitive ability. However, in conditions of perceived financial scarcity, a helpful nudge can potentially backfire. This work has the policy implication that these vulnerable groups should be the focus on the regulation of potentially harmful nudges through marketing and advertising and of helpful nudges through careful policy design to avoid such backfire effect.