Article dans une revue: Herein we explore whether a solemn oath can eliminate hypothetical bias in a voting referenda, a popular elicitation mechanism promoted in non-market valuation exercises for its incentive compatibility properties. First, we reject the null hypothesis that a hypothetical bias does not exist. Second, we observe that people who sign an oath are significantly less likely to vote for the public good in a hypothetical referenda. We complement this evidence with a self-reported measure of honesty which confirms that the oath increases truthfulness in answers. This result opens interesting avenues for improving the elicitation of preferences in the lab and beyond.