The primary strategy that is used to fight illegal business cartels is one of general deterrence. In practice this means that authorities investigate and sanction cartels to show other business executives that it does not pay to form a cartel. The fines are meant to confer the severity of the fines and the risk of getting caught. However, this approach simplifies people as purely driven by incentives and does not take into account the complex nature of human perception and decision-making. Through several survey studies and experimental research among business executives, this dissertation provides insight into the drivers of cartel behaviour and the effects of fine communication. The findings in this dissertation show that norms toward cartels and knowledge of the law are potentially much more important in the fight against cartels than deterrence perceptions. Firstly, survey research among Dutch business executives showed that norms toward cartel behaviour and knowledge of the law have a much stronger relationship to intention to show cartel behaviour than perceptions of fine severity and risk of detection. Secondly, the results of two studies on specific fines show that competition fines are likely primarily effective in improving knowledge of the law and potentially also in improving the norm toward cartel behaviour. The studies show no evidence that specific fines influence perceptions of fine severity or detection risk. Finally, two experimental studies on fine messaging showed that deterrence frames can be used to improve business executives’ norms toward cartel behaviour, but are unlikely to affect their perceptions of fine severity or detection risk. These results provide a more complete picture of cartel behaviour and give competition authorities new insights to improve their supervision and look beyond a sole focus on deterrence.