Plausibility matters: A challenge to Gilbert's "Spinozan" account of belief formation.

Most of the claims we encounter in real life can be assigned some degree of plausibility, even if they are new to us. On Gilbert's (1991) influential account of belief formation, whereby understanding a sentence implies representing it as true, all new propositions are initially accepted, before any assessment of their veracity. As a result, plausibility cannot have any role in initial belief formation on this account. In order to isolate belief formation experimentally, Gilbert, Krull, and Malone (1990) employed a dual-task design: if a secondary task disrupts participants' evaluation of novel claims presented to them, then the initial encoding should be all there is, and if that initial encoding consistently renders claims 'true' (even where participants were told in the learning phase that the claims they had seen were false), then Gilbert's account is confirmed. In this pre-registered study, we replicate one of Gilbert et al.'s (1990) seminal studies ("The Hopi Language Experiment") while additionally introducing a plausibility variable. Our results show that Gilbert's 'truth bias' does not hold for implausible statements - instead, initial encoding seemingly renders implausible statements 'false'. As alternative explanations of this finding that would be compatible with Gilbert's account can be ruled out, it questions Gilbert's account.