Increased belief in one conspiracy theory leads to increased belief in others over time

The monological belief system model (Goertzel, 1994) suggests that believing in one conspiracy theory causes a person 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) lead to increased belief in other conspiracies at the next wave. However, the estimated effects were small.