Over the past two decades, research about the role of oxytocin (OT) in human behavior has grown exponentially. However, a unified theory of OT effects has yet to be developed. Relatedly, growing concerns about the robustness of conclusions drawn in the field have been raised. The current article contributes to this debate by reporting on and discussing key conclusions from a systematic review of published studies addressing the interactive effects of intranasal OT (IN-OT) administration on psychosocial outcomes in a healthy population. The review indicates that (a) tested interactive IN-OT effects were highly heterogeneous; (b) for most published interactions, no replication was attempted; (c) when attempted, replications were largely unsuccessful; (d) significance was unrelated to sample size; (e) statistical power was critically low and unrelated to the rate of significant results; and (f) research practices were characteristic of an exploratory approach. This concerning state of affairs makes it virtually impossible to tease apart true from false interactive IN-OT effects. We provide constructive directions on the basis of this observation and positive predictive value simulations for future research that should help extract true effects from noise and move the IN-OT field forward.

A single exposure to statements is typically enough to increase their perceived truth. This Truth-by-Repetition (TBR) effect has long been assumed to occur only with statements whose truth value is unknown to participants. Contrary to this hypothesis, recent research found a TBR effect with statements known to be false. Of note, a recent model even posits that repetition could increase the perceived truth of highly implausible statements. As for now, however, no empirical evidence has reported a TBR effect for highly implausible statements. Here, we reasoned that one may be found provided a sensitive truth measure is used and statements are repeated more than just once. In a preregistered experiment, participants judged the truth of highly implausible statements on a 100-point scale, and these statements were either new to them or had been presented five times before the judgment task. We observed a TBR effect: truth judgments were higher for repeated statements than for new ones -even if all statements were still judged as false.Exploratory analyses additionally suggest that all participants were not equally prone to this TBR effect: about half the participants showed no or even a reverse effect. Overall, the results provide direct empirical evidence to the claim that repetition can increase perceived truth even for highly implausible statements, although not equally so for all participants and not to the point of making the statements look true.

Corneille et al. (2020) found that repetition increases judgments that statements have been used as fake news on social media. They also found that repetition increases truth judgments and decreases falsehood judgments (i.e., two instantiations of the truth-by-repetition effect). These results supported an ecological explanation of the truth-by-repetition effect better than two alternative accounts. However, the first author of the present article found unsuspected programming issues in Corneille et al.'s experiments. These programming issues introduced confounds that may have been responsible for the results. To estimate whether Corneille et al.'s main findings and claims hold when correcting these issues, the current team agreed on two highpowered preregistered replications of Corneille et al.'s experiments (N total = 540). The results replicate Corneille et al.'s findings, which are more consistent with an ecological account of repetition effects on judgment than the alternative accounts tested in the original publication.

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Why individuals believe in conspiracy theories is a theoretical question with practical implications. Research has mostly focused on individual differences and motives, and we therefore know little about how manipulated variables may affect conspiracism. Based on the truth effect literature, which has demonstrated that statements already seen (or perceived as already seen) are more likely to be judged as true than new ones, we hypothesized that repeated exposure to conspiracy theories can increase the likelihood of their being believed. If this were confirmed, it would help to challenge truth ambiguity as a boundary condition of the truth effect. As an initial test of this hypothesis, we analyzed data from two surveys conducted in representative samples of the French population (IFOP, 2017, 2019). Participants indicated both their adherence to and recognition of conspiracy statements in each survey, corresponding to a total of 17 widespread conspiracy theories. We found a truth effect in each dataset, although with different magnitudes. This truth effect was positively associated with a conspiracy mentality. These results suggest that individuals may form beliefs in conspiracy theories through the truth effect, and lend further weight to the reassessment of truth ambiguity as a boundary condition of the truth effect.