The COVID-19 pandemic has made the world seem less predictable. Such crises can lead people to feel that others are a threat. Here, we show that the initial phase of the pandemic in 2020 increased individuals’ paranoia and made their belief updating more erratic. A proactive lockdown made people’s belief updating less capricious. However, state-mandated mask-wearing increased paranoia and induced more erratic behaviour. This was most evident in states where adherence to mask-wearing rules was poor but where rule following is typically more common. Computational analyses of participant behaviour suggested that people with higher paranoia expected the task to be more unstable. People who were more paranoid endorsed conspiracies about mask-wearing and potential vaccines and the QAnon conspiracy theories. These beliefs were associated with erratic task behaviour and changed priors. Taken together, we found that real-world uncertainty increases paranoia and influences laboratory task behaviour.

COVID-19 大流行使世界看起来更难以预测。这种危机会使人们感到他人是一种威胁。在这里，我们表明，2020 年大流行的初始阶段增加了个人的偏执，并使他们的信念更新更加不稳定。主动封锁使人们的信念更新不那么反复无常。然而，国家规定的戴口罩增加了偏执狂并引发了更多古怪的行为。这在遵守戴口罩规则较差但遵守规则通常更为普遍的州最为明显。对参与者行为的计算分析表明，偏执狂较高的人预计任务会更加不稳定。更偏执的人支持关于戴口罩和潜在疫苗的阴谋论以及 QAnon 阴谋论。这些信念与不稳定的任务行为和改变的先验有关。综上所述，我们发现现实世界的不确定性会增加偏执并影响实验室任务行为。