Although legal contexts are subject to biased reasoning and decision making, to identify and test debiasing techniques has largely remained an open task. We report on experimentally deploying the technique “giving reasons pro et contra” with professional (N = 239) and lay judges (N = 372) at Swedish municipal courts. Using a mock legal scenario, participants assessed the relevance of an eyewitness’s previous conviction for his credibility. On average, both groups displayed low degrees of bias. We observed a small positive debiasing effect only for professional judges. Strong evidence was obtained for a relation between profession and relevance-assessment: Lay judges seemed to assign a greater importance to the prior conviction than professional judges did. We discuss challenges for future research, calling other research groups to contribute additional samples.