Conspiracy theories are part of mainstream public life, with the potential to undermine governments, promote racism, ignite extremism and threaten public health efforts. Psychological research on conspiracy theories is booming, with more than half of the academic articles on the topic published since 2019. In this Review, we synthesize the literature with an eye to understanding the psychological factors that shape willingness to believe conspiracy theories. We begin at the individual level, examining the cognitive, clinical, motivational, personality and developmental factors that predispose people to believe conspiracy theories. Drawing on insights from social and evolutionary psychology, we then review research examining conspiracy theories as an intergroup phenomenon that reflects and reinforces societal fault lines. Finally, we examine how conspiracy theories are shaped by the economic, political, cultural and socio-historical contexts at the national level. This multilevel approach offers a deep and broad insight into conspiracist thinking that increases understanding of the problem and offers potential solutions. Subject terms: Human behaviour, Psychology Conspiracy theories have the potential to undermine governments, promote racism, ignite extremism and threaten public health efforts. In this Review, Hornsey et al. synthesize the literature on factors that shape conspiracy beliefs at the individual, intergroup and national level. status released display-pdf yes is-in-collection-domain yes is-olf no is-manuscript no is-preprint no is-journal-matter no is-scanned no is-retracted no Accepted 2022 Oct 25; Issue date 2023. The developmental approach Finally, although there has been progress in creating measures of conspiracy belief suitable for children and adolescents 110 , there has been little research on how conspiracy beliefs develop across the lifespan. Some have suggested that developmental experiences can affect willingness to believe conspiracy theories owing to their role in shaping attachment styles. For example, one study found that conspiracy beliefs were associated with anxious but not avoidant attachment 111 . However, In line with this perspective, an individual’s conspiracy belief is partly influenced by the extent to which other group members also believe that conspiracy theory 116 . Furthermore, social identity theory is based on the simple observation that there is a general bias towards wanting to think the best of groups to which one belongs 117 , 118 . A simple extrapolation from this notion is that people might be more likely to believe outgroups are capable of sinister acts of collusion compared with ingroups. Examples of this phenomenon abound. Finally, followers of New Age spiritual practices are more likely than Christian people to believe the conspiracy that the Catholic Church kept secret Jesus’ marriage to Mary Magdalene, and that there is a secret organization protecting the ‘holy lineage’ that flowed from that union 122 . Clearly, group loyalties powerfully affect which conspiracy theories people are willing to believe 123 – 125 to the point that one’s choice of conspiracy theories can signal group loyalties 126 . Second, antisemitic conspiracy theories are a proximal precursor for violence. Historically, stories of secret Jewish plots have been central features of propaganda campaigns that have precipitated ethnic cleansing. The conspiracy theory that Jewish people are plotting to displace Christian European populations has become a central feature of white supremacist ideology in the West, and appears in the manifestos of numerous domestic terrorists 159 . Research in Poland shows that, of all the varieties of antisemitic belief, conspiracy beliefs were the strongest predictors of antisemitic behavioural intentions 20 . Third, antisemitic conspiracy beliefs are not spontaneously formed by individual actors: they are constructed and disseminated by provocateurs as elements of organized campaigns designed to prepare people for violence (such as the Protocols of the Elders of Zion pamphlet) 160 . This underscores that conspiracy theories are not just factoids discovered by vulnerable minds. They can also be features of infrastructures of misinformation that are authored, cultivated and designed with specific (and malicious) intent. On the other hand, the relationship between conspiracy belief and the Democracy Index might be underestimated, because participants from authoritarian nations might be wary of revealing true levels of suspicion about the actions and motives of elite institutions within their country. Integrating levels of understanding A critical mass of research exists on drivers of conspiracy beliefs at micro (individual), meso (intergroup) and macro (national) levels of analysis, but these typically operate as discrete bodies of literature. Populist politicians frame themselves as representatives of ordinary people who will work within the secretive and corrupt political system to revolutionize it, effectively destroying conspiracies 163 . It is therefore not surprising that populist attitudes and support for populist politicians are reliably associated with conspiracy beliefs 164 , 165 . For example, people who support former populist President Donald Trump are significantly more vaccine-hesitant than other Americans because they are more prone to conspiracy beliefs 166 . The success of populist politicians internationally in the past decade has prompted commentary that the world is entering an era of politics where the usual trust algorithm is inverted: instead of representing the political system, politicians receive support by affirming suspicions that the political system is untrustworthy and secretive 167 . This phenomenon is a reminder that conspiracy beliefs do not always emerge spontaneously at the individual level; they can also be manufactured at the macro level by political operatives and the media that support them.