Dishonest behaviours such as tax evasion impose significant societal costs. Ex ante honesty oaths—commitments to honesty before action—have been proposed as interventions to counteract dishonest behaviour, but the heterogeneity in findings across operationalizations calls their effectiveness into question. We tested 21 honesty oaths (including a baseline oath)—proposed, evaluated and selected by 44 expert researchers—and a no-oath condition in a megastudy involving 21,506 UK and US participants from Prolific.com who played an incentivized tax evasion game online. Of the 21 interventions, 10 significantly improved tax compliance by 4.5 to 8.5 percentage points, with the most successful nearly halving tax evasion. Limited evidence for moderators was found. Experts and laypeople failed to predict the most effective interventions, though experts’ predictions were more accurate. In conclusion, honesty oaths were effective in curbing dishonesty, but their effectiveness varied depending on content. These findings can help design impactful interventions to curb dishonesty.

逃税等不诚实行为会带来巨大的社会成本。事前诚实誓言——在行动前承诺诚实——已被提议作为抵制不诚实行为的干预措施，但不同操作化结果的异质性使其有效性受到质疑。我们在一项大型研究中测试了 21 个诚实誓言（包括基线誓言）——由 44 名专家研究人员提出、评估和选择——以及一项不宣誓条件，该研究涉及来自 Prolific.com 的 21,506 名英国和美国参与者，他们在网上玩了激励性逃税游戏。在 21 项干预措施中，有 10 项将税收合规性显著提高了 4.5 至 8.5 个百分点，其中最成功的逃税率几乎减半。发现版主的证据有限。专家和非专业人士未能预测最有效的干预措施，尽管专家的预测更准确。总之，诚实誓言在遏制不诚实方面是有效的，但它们的有效性因内容而异。这些发现可以帮助设计有影响力的干预措施来遏制不诚实行为。