BACKGROUND: If excessive mistrust - for example, holding conspiracy beliefs or experiencing paranoia - is widespread then people should notice it in others. We aimed to assess the degree to which the general population had observed excessive mistrust. Paranoia is a particular form of excessive mistrust. How people understand paranoia, and therefore react to it, could affect its persistence. We also aimed to learn how the general population views paranoia. METHODS: A non-probability online survey was conducted in May 2024 with 1,036 UK adults, quota sampled to match the population for age, gender, ethnicity, income, and region. Nine examples of excessive mistrust in others were presented. Knowledge of paranoia was also assessed. RESULTS: Participants (n = 698, 67.4%) had most commonly encountered a person 'Saying that they would not get a COVID-19 vaccination because of concerns about the real motivation behind the vaccine rollout'. Least commonly encountered by participants (n = 328, 31.7%) was a person 'Thinking that others are targeting them in order to bully or exploit them, and so isolating from the world and refusing to leave their home'. A total of 854 (82.4%) participants had observed at least one form of excessive mistrust in the past year, most frequently in friends. More mistrustful participants were more likely to observe mistrustful behaviors. Participants endorsed multiple causes of paranoia, with the most endorsed causes being worry and illicit drugs. CONCLUSIONS: The large majority of people have encountered others, primarily individuals they know, exhibiting excessive mistrust. Public understanding of paranoia varies greatly, with diverse definitions and perceived causes.