Does Developing a Belief in One Conspiracy Theory Lead a Person to be More Likely to Believe in Others?

The monological belief system model suggests that-for at least a subset of people-developing a belief in one conspiracy theory will cause them to be more likely to believe in others. This model has been influential in the literature, but its core causal hypothesis has never been credibly tested. We therefore tested it in two longitudinal studies. Study 1 used a sample from New Zealand and Australia (N = 498), with 7 monthly waves. Study 2 (preregistered) used a sample from New Zealand, Australia and the United Kingdom (N = 978), with 13 monthly waves. We applied random intercept cross-lagged panel models, permitting a credible causal identification strategy, albeit we cannot rule out time-varying confounds. We find that increased belief in a conspiracy theory at one wave did (on average) predict increased belief in other conspiracies at the next wave, although the estimated coefficients were small.