Although communicating so-called ‘dynamic norms’ may promote sustainable consumption behavior, the present study challenges the assumption that their impact is inevitably positive. On the contrary, it seeks to expand current research by establishing when and why sources with vested motives may result in a backfiring effect of dynamic norms. Our experiment (N=352) investigates consumers' intentions to reduce their meat consumption after reading norm messages from different sources (i.e., a researcher, vegan activist, company representative). In line with the predictions, dynamic norm messages from a researcher led to more sustainable consumption choices than identical messages communicated by a vegan activist or company representative while controlling for gender. Concerning the underlying theoretical mechanisms, a perceived moral superiority and ensuing psychological reactance sequentially explained the backfiring effect for the vegan activist. The backfiring effect for the company representative was mediated by a perceived pro-self-motivation that again resulted in elevated consumer reactance. Overall, the present findings establish the crucial importance of a nudge's source that either fosters or undermines the effectiveness of a dynamic norm message in eliciting sustainable behavior. The results not only contribute to the academic debate on nudging, but can also inform practitioners to choose the most powerful sender (e.g. academics) when designing sustainable norm interventions.