“60 Years Later: A Replication Study of McGuire’s First Inoculation Experiment”, Marieke L. Fransen, Saar Mollen, Stephan A. Rains, Enny Das, Ivar Vermeulen2023-09-13 (cognitive bias)⁠:

[data/pre-registration] Inoculation theory was introduced 60 years ago, after McGuire & Papageorgis1961 published their first study on how resistance to persuasion can be induced. They demonstrated that people who are pre-exposed to weakened arguments against an attitude or position they currently hold (ie. inoculated) are less affected by a subsequent strong counter-attitudinal message than people who are pre-exposed to arguments consistent with their attitude (ie. supportive defense treatment) or to no arguments. Although these results substantially impacted both science and practice on a general level, rigid tests of the key theoretical propositions are lacking.

We conducted a high-powered replication study (n = 679) and found that:

an inoculation treatment is more effective in increasing resistance toward persuasion compared to a supportive defense treatment and a no-treatment control condition.

Our results were mostly consistent with McGuire and Papageorgis’s original work.

[Keywords: replication, inoculation theory, persuasion, resistance]

…The estimated original effect size of the difference between the inoculation and supportive condition falls within the confidence interval of the observed effect size. This is not the case for the difference between the inoculation and the control condition. The effect size for this effect, however, is nearly identical to the effect size observed in the meta-analysis on inoculation studies (Banas & Rains2010). These findings lend convincing support for the assumptions of inoculation theory. With this replication study, effect sizes of the original design with the original materials are identified making it possible to include these results in future meta-analyses. After more than 60 years, the same effects were produced. This shows the robustness of the theory and demonstrates that more recent studies that base their assumptions on inoculation theory are building from a solid foundation.

…interestingly, in our pilot test we observed that all the truisms that were used in the original study could not be classified as truisms when considering 13 as a minimum belief score. Mean belief scores on the truisms ranged 6.25–11.73, and the more contemporary topics that we tested as truisms scored 7.07–13.64 with relatively high standard deviations. This could indicate that people today are more critical and hold more diverse beliefs about the topics that were previously considered as true by most people. This attests to the idea that cultural truisms are fluid and change over time. We also see this in the data of our replication study; unexpectedly many participants had a belief score lower than 10 on one or both topics at T₀. This might be a result of the overwhelming information that is available today and the many different opinions, misinformation, and disinformation people are confronted with daily.

…Like the results in the original study, it was found that the attitude scores at T₀ already differed between treatments. Participants who received the inoculation treatment showed lower belief scores, especially when compared with participants who received the supportive treatment. This shows that it is difficult for people to indicate their beliefs without taking the information that they just read into account, even when they are specifically asked to do so.