<p>Countries vary greatly in conspiracy beliefs. However, little research has examined how group-based traits like outgroup mistrust and psychological reactance (a tendency to resist external control and assert autonomy through nonconformity) interact with cultural context to shape conspiratorial thinking. We examined the role of these traits in explaining variation in health conspiracy beliefs and conspiracy mentality across three countries with different historical and socio-cultural contexts. We analysed demographically representative data from over 4500 participants in the UK, US, and Turkey using hierarchical linear models. Outgroup mistrust and psychological reactance emerged as the strongest predictors of conspiracy beliefs, with reactance having the largest effect on conspiracy mentality and both traits strongly predicting belief in health conspiracies. Control variables including belief in science, political orientation, religiosity, and demographic factors had minimal effects. Turkish participants had the highest conspiracy mentality and health conspiracy scores, along with higher levels of outgroup mistrust and reactance, while UK participants had the lowest. Belief in health conspiracies and conspiracy mentality were strongly associated with vaccine hesitancy across all three countries, with the highest hesitancy in Turkey. Moderation analyses revealed that outgroup mistrust was a stronger predictor of health conspiracy beliefs in Turkey, while reactance had a greater impact in the UK and the US. These results suggest that outgroup mistrust and signalling non-conformity play a crucial role in conspiracy beliefs and variations in their levels across cultures. Based on these insights and a cultural evolutionary perspective, we propose strategies to address conspiracy beliefs and vaccine hesitancy.</p>