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Social norms messaging campaigns are increasingly used to influence human behavior, with social science research generally finding that they have modest but meaningful effects. One aspect of these campaigns in practice has been the inclusion of injunctive norms messaging, designed to convey a social judgement about one's behaviors (often in the form of encouraging or discouraging language, or a visual smiley or frowny face). While some prominent research has provided support for the use of such messaging as a tool for positive behavior change, causal evidence on the effect of injunctive norms messaging as a motivator (as opposed to just one part of a multifaceted messaging campaign) is limited. This paper presents a field experiment on water conservation behavior conducted by an organization in California, involving over 40,000 households, which provides some of the most precise evidence to date regarding the effect of injunctive norms on decision making. I find that not only do injunctive norms encourage conservation behavior, there is also no evidence that they discourage individuals from further attending norms messaging-regardless of whether the social judgement conveyed is negative or positive. Taken together, this suggests that injunctive norms are a useful tool in "nudge"-style campaigns tackling behavior change.