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Four studies (total n = 961) developed and validated the Adolescent Conspiracy Beliefs Questionnaire (ACBQ). Initial items were developed in collaboration with teachers. An exploratory factor analysis (Study 1, n = 208, aged 11-14) and a student focus group (N = 3, aged 11) enabled us to establish the factor structure of a 9-item scale. This was replicated via confirmatory factor analysis in Study 2 (N = 178, aged 11-17), and the scale displayed good convergent (i.e., relationship with paranoia and mistrust) and discriminant validity (i.e., no relationship with extraversion). Study 3a (N = 257) further tested convergent validity with a sample of 18-year-olds (i.e., relationship with adult-validated measures of conspiracy beliefs) and demonstrated strong test-retest reliability. Study 3b (N = 318) replicated these findings with a mixed-age adult sample. The ACBQ will allow researchers to explore the psychological antecedents and consequences of conspiracy thinking in young populations. Statement of contribution What is already known on this subject? Conspiracy theories can have a significant impact on societal issues. Despite their social importance, it is difficult to examine conspiracy beliefs across the lifespan. Conspiracy belief measures are designed for adults and cannot capture the beliefs of adolescents. What does this study add? We have developed and validated a novel measure of conspiracy beliefs suitable for adolescents. The measure will be invaluable for learning how conspiracy beliefs change across the lifespan. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Anti-vaccine conspiracy beliefs among parents can reduce vaccination intentions. Parents’ beliefs in anti-vaccine conspiracy theories are also related to their perceptions of other parents’ conspiracy beliefs. Further, research has shown that parents hold misperceptions of anti-vaccine conspiracy belief norms: UK parents over-estimate the anti-vaccine conspiracy beliefs of other parents. The present study tested the effectiveness of a Social Norms Approach intervention, which corrects misperceptions using normative feedback, to reduce UK parents’ anti-vaccine conspiracy beliefs and increase vaccination intentions. At baseline, 202 UK parents of young children reported their personal belief in anti-vaccine conspiracy theories, future intentions to vaccinate, and their perceptions of other UK parents’ beliefs and intentions. Participants were then randomly assigned to a normative feedback condition (n = 89) or an assessment-only control condition (n = 113). The normative feedback compared participants’ personal anti-vaccine conspiracy beliefs and perceptions of other UK parents’ beliefs with actual normative belief levels. Parents receiving the normative feedback showed significantly reduced personal belief in anti-vaccine conspiracy beliefs at immediate post-test. As hypothesised, changes in normative perceptions of anti-vaccine conspiracy beliefs mediated the effect of the intervention. The intervention, did not directly increase vaccination intentions, however mediation analysis showed that the normative feedback increased perceptions of other parents’ vaccination intentions, which in turn increased personal vaccination intentions. No significant effects remained after a six-week follow-up. The current research demonstrates the potential utility of Social Norms Approach interventions for correcting misperceptions and reducing anti-vaccine conspiracy beliefs among UK parents. Further research could explore utilising a top-up intervention to maintain the efficacy.

Conspiracy beliefs are widespread and can have detrimental consequences. As perceived social norms can exert a powerful influence on individuals, we investigated the relationship between perceived conspiracy belief norms and personal endorsement, and whether others’ conspiracy belief is overestimated. In Study 1, UK university students ( N = 111) completed measures of their personal conspiracy beliefs and estimations of others’ beliefs (an in-group and an out-group they chose, and a prescribed in-group). Perceived in-groups’ belief strongly predicted personal conspiracy belief; perceived out-group’s belief did not. Studies 2 and 3 replicated these findings in a British community sample ( N = 177) and in a UK parent sample ( N = 197), focusing on antivaccine conspiracy theories. All studies demonstrated that people overestimate the conspiracy beliefs of others. This is the first demonstration of the association between perceived in-group conspiracy belief social norms and individuals’ personal conspiracy beliefs. Interventions challenging misperceived norms could be effective in reducing conspiracy beliefs.

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