In this research, we investigated the social influence of newspaper headlines on beliefs on various social, political, and economic issues, including belief in conspiracy theories. Building on the seminal study by Gruenfeld and Wyer (1992), we examined how denials and affirmations printed in a credible source (e.g., a newspaper considered to be serious) versus a less credible source (e.g., a free newspaper) affected readers’ beliefs. In this computer-based study, participants were asked to rate the plausibility of 24 newspaper statements (eight of which were related to conspiracy theories), first without any mention of a newspaper and then with the newspapers mentioned as sources. The results showed the general effects associated with the degree of informativeness of the statements. We discuss these effects in terms of the boomerang effect (i.e., opinion change in the direction opposite to that of the opinion given in the headline). We also found that the participants judged the official versions of various events to be more plausible than the conspiracy theory versions of the same events.