Paranoia is the belief that harm is intended by others. It may arise from selective pressures to infer and avoid social threats, particularly in ambiguous or changing circumstances. We propose that uncertainty may be sufficient to elicit learning differences in paranoid individuals, without social threat. We used reversal learning behavior and computational modeling to estimate belief updating across individuals with and without mental illness, online participants, and rats chronically exposed to methamphetamine, an elicitor of paranoia in humans. Paranoia is associated with a stronger prior on volatility, accompanied by elevated sensitivity to perceived changes in the task environment. Methamphetamine exposure in rats recapitulates this impaired uncertainty-driven belief updating and rigid anticipation of a volatile environment. Our work provides evidence of fundamental, domain-general learning differences in paranoid individuals. This paradigm enables further assessment of the interplay between uncertainty and belief-updating across individuals and species.

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. ArticlesNATure HumAN BeHAVIOur (BF 10 = 0.163, strong evidence for the null hypothesis) or reversals achieved (BF 10 = 0.210, strong evidence for the null hypothesis) between social and non-social tasks. Computational modelling. Probabilistic reversal learning involves decision-making under uncertainty. The reasons for decisions may not be manifest in simple counts of choices or errors. By modelling Reward probability (%) 10

The 2019 coronavirus (COVID-19) pandemic has made the world seem unpredictable. During such crises we can experience concerns that others might be against us, culminating perhaps in paranoid conspiracy theories. Here, we investigate paranoia and belief updating in an online sample (N=1,010) in the United States of America (U.S.A). We demonstrate the pandemic increased individuals’ self-rated paranoia and rendered their task-based belief updating more erratic. Local lockdown and reopening policies, as well as culture more broadly, markedly influenced participants’ belief-updating: an early and sustained lockdown rendered people’s belief updating less capricious. Masks are clearly an effective public health measure against COVID-19. However, state-mandated mask wearing increased paranoia and induced more erratic behaviour. Remarkably, this was most evident in those states where adherence to mask wearing rules was poor but where rule following is typically more common. This paranoia may explain the lack of compliance with this simple and effective countermeasure. Computational analyses of participant behaviour suggested that people with higher paranoia expected the task to be more unstable, but at the same time predicted more rewards. In a follow-up study we found people who were more paranoid endorsed conspiracies about mask-wearing and potential vaccines – again, mask attitude and conspiratorial beliefs were associated with erratic task behaviour and changed priors. Future public health responses to the pandemic might leverage these observations, mollifying paranoia and increasing adherence by tempering people’s expectations of other’s behaviour, and the environment more broadly, and reinforcing compliance.

55Paranoia is the belief that harm is intended by others. It may arise from selective pressures to infer and avoid 56 social threats, particularly in ambiguous or changing circumstances. We propose that uncertainty may be 57 sufficient to elicit learning differences in paranoid individuals, without social threat. We used reversal learning 58 behaviour and computational modelling to estimate belief updating across individuals with and without mental 59 illness, online participants, and rats exposed to chronic methamphetamine, an elicitor of paranoia in humans. 60Paranoia is associated with a strong but immutable prior on volatility, accompanied by elevated sensitivity to 61 perceived changes in the task environment. Methamphetamine exposure in rats recapitulates this impaired 62 uncertainty-driven belief updating and rigid anticipation of a volatile environment. Our work provides evidence 63 of fundamental, domain-general learning differences in paranoid individuals. This paradigm enables further 64 assessment of the interplay between uncertainty and belief-updating across individuals and species. 66 67Paranoia & Belief Updating 3 Paranoia is excessive concern that harm will occur due to deliberate actions of others 1 . It manifests along a 68 continuum of increasing severity 2-5 . Fleeting paranoid thoughts prevail in the general population 6 . A survey of 69 over 7,000 individuals found that nearly 20% believed people were against them at times in the past year; 70 approximately 8% felt people had intentionally acted to harm them 4 . At a national level, paranoia may fuel 71 divisive ideological intolerance. Historian Richard Hofstadter famously described catastrophizing, context 72 insensitive political discourse as the 'paranoid style': 73 "The paranoid spokesman sees the fate of conspiracy in apocalyptic terms-he traffics in the birth 74 and death of whole worlds, whole political orders, whole systems of human values. He is always 75 manning the barricades of civilization. He constantly lives at a turning point." 7 76 77 At its most severe, paranoia manifests as rigid beliefs known as delusions of persecution. These delusions 78 occur frequently in psychotic illness, including nearly 90% of first episode patients 8 . However, paranoid beliefs 79 are common across psychiatric and neurologic disorders, such as anxiety 9 , depression 10 , epilepsy 11 , and 80 Alzheimer's disease 12 . Psychostimulants elicit severe paranoid states. Methamphetamine evoked new 81paranoid ideation in nearly half of 274 respondents, particularly after repeated exposure (86%) or escalating 82 dose (68%) 13 . Of those who became paranoid, the majority engaged in evasive defence strategies (hiding or 83 fleeing), but 37% obtained weapons, and 15% attacked others. There is a clear need to better manage 84 paranoia, and to understand and address its broader societal impact. 86Paranoia has thus far defied explanation in mechanistic terms, either at the levels of behaviour or brain 87 function. Obvious links with fear processing and social ...