This paper documents the existence of a 'formality effect' in government communications. Across three online studies and three field experiments in different policy contexts (total N = 67,632), we show that, contrary to researcher and practitioner predictions, formal government communications are more effective at influencing resident behaviour than informal government communications. In exploring mechanisms, we show that formality operates as a heuristic for credibility and importance. Recipients view the source of a formal letter as more competent and trustworthy, and view the request itself as more important to take action on, despite no evidence of change in comprehension or in perceived ease of taking action. These findings have immediate implications for government communicators and open the door for a renewed focus on how the design and presentation of information impacts behaviour.