Study reveals judges handed down harsher sentences to Moroccan-Dutch defendants after high-profile assassination

Authored by Prof. Olivier Marie (Erasmus School of Economics), Dr Kyra Hanemaaijer (University of Gothenburg), and Dr Nadine Ketel (VU Amsterdam), the study provides rare, data-driven evidence of how “salience shocks” (sudden increases in the public visibility of certain minority groups) can influence judicial decision-making when cognitive shortcuts replace impartial reasoning.

Using high-quality administrative data from Statistics Netherlands covering every stage of the criminal justice process, the authors analysed the aftermath of Wiersum’s assassination, which was widely linked in Dutch media to the so-called 'Mocro Maffia', a criminal network involving Moroccan-Dutch individuals.

When public attention shapes justice

The findings reveal that judicial sentencing became significantly harsher following the assassination. Judges increased prison sentence lengths for defendants of Moroccan descent by 72 percent (equivalent to roughly three additional months on average). In contrast, the likelihood of receiving a prison sentence did not change, nor did police or prosecutorial decisions, indicating that bias emerged only in sentence length, the stage of the process where judges exercise the most discretion. The study also shows that judges with more experience handling case involving Moroccan defendants were less affected by the salience shock, indicating that professional exposure can help mitigate bias. Finally, the researchers found that these disparities persisted through the appeals process, as higher courts did not correct the harsher sentences imposed in the aftermath of the event.

Professor of Labour Economics, Olivier Marie, explains: ‘Our results show that even in a modern, rule-based justice system, heightened attention toward a particular minority can subconsciously influence judicial decisions. It highlights the importance of awareness and training in mitigating bias during moments of societal tension’.

A broader European perspective

While studies of judicial bias have often focused on the United States, this research provides one of the first comprehensive analyses of such effects within a European criminal justice system. It demonstrates how ethnic and migrant backgrounds can shape outcomes, even in a context with strong institutional safeguards and norms of equality before the law.