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.

Baruch Ivcher School of Psychology Reichman University Herzliya Israel

Batten School of Leadership and Public Policy University of Virginia Charlottesville VA USA

Center for Trustworthy Data Science and Security University of Duisburg Essen Duisburg Germany

CES Université Paris 1 Panthéon Sorbonne Paris France

CESifo Munich Munich Germany

Department of Business Administration Ben Gurion University of the Negev Beersheba Israel

Department of Economics and Business Economics Aarhus University Aarhus Denmark

Department of Economics Johannes Kepler University Linz Linz Austria

Department of Economics Management and Quantitative Methods University of Milan Milan Italy

Department of Management Aarhus University Aarhus Denmark

Department of Management Prague University of Economics and Business Prague Czech Republic

Department of Philosophy and Religious Studies NTNU Trondheim Trondheim Norway

Department of Philosophy University of Milan Milan Italy

Department of Psychology and Behavioural Sciences Aarhus University Aarhus Denmark

Department of Psychology Bielefeld University Bielefeld Germany

Department of Psychology University of Kaiserslautern Landau Landau Germany

Department of Psychology University of Milan Bicocca Milan Italy

Department of Strategy and Management Norwegian School of Economics Bergen Norway

Faculty of Law Bar Ilan University Ramat Gan Israel

Faculty of Social Sciences Charles University Prague Czech Republic

Federal University of Applied Administrative Sciences Berlin Germany

Federmann School of Public Policy Hebrew University of Jerusalem Jerusalem Israel

Fresenius University Hamburg Germany

Fuqua School of Busisness Duke University Durham NC USA

IESE Business School University of Navarra Barcelona Spain

Institute of Economics Ulm University Ulm Germany

Institute of Environmental Planning Leibniz University Hannover Hannover Germany

Institute of Psychology University of Wrocław Wrocław Poland

Jagiellonian University Krakow Poland

Max Planck Institute for Human Development Berlin Germany

Max Planck Institute for the Study of Crime Security and Law Freiburg Germany

Paris School of Economics Paris France

Questrom School of Business Boston University Boston MA USA

School of Psychology University of Plymouth Plymouth UK

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