When organizations want their employees to adopt behaviors that advance prosocial and instrumental aims, which motive should they express? A groundswell of recent work suggests that highlighting prosocial actions inspires and motivates employees. Building on this work, we embed a field experiment in the context of an organizational change initiative (Study 1). A large university sought to change the behavior of administrative employees who purchase office supplies, encouraging them to place orders of at least $50, referred to as “bundling.” We exploit the fact that the organization could justify the same behavior in contrasting ways. We randomly assign employees to view either a prosocial (“limiting pollution”), instrumental (“limiting costs”), or mixed motive (“limiting pollution and limiting costs”) for caring about bundling each time they access the organization’s procurement system. We then evaluate changes in employees’ behavior by comparing a six-month baseline to a six-month experimental period, covering 10,169 purchases in 556 offices. Contrary to expectations from related research, the instrumental motive was most effective for changing behavior, leading to significantly more bundling than the prosocial motive. Two follow-up vignette experiments probe theoretical mechanisms. They indicate that an instrumental motive seems more genuine (i.e., reflecting the organization’s true motive) than a prosocial motive (Study 2) and that seeming genuine increases individuals’ intention to bundle (Study 3). This research reveals that prosocial justifications can be less effective than instrumental ones and suggests that perceptions of genuineness may shape the effectiveness of behavioral change efforts in organizations. This paper was accepted by Yuval Rottenstreich, decision analysis.