Do claims about certainty make estimates less certain?

Predictions and estimates are sometimes qualified as certain. This epistemic marker occupies a privileged position at the top of scales of verbal probability expressions, reflecting probabilities close to 1. But such statements have rarely been compared to plain, unqualified statements in which certainty is not mentioned. We examined in nine studies (N = 2784) whether statements explicitly claimed to be certain are perceived as (1) more (or less) credible, (2) more (or less) precise, and (3) more (or less) strongly based upon evidence, compared to plain, unmarked declarative statements. We find, in apparent contrast with assumptions made by the standard scales, that “certain” are often judged to be less trustworthy, less reliable, and held with lower confidence than unmarked statements. Plain, declarative statements are further assumed to be more precise, while certainty implies that more extreme outcomes are possible. When it is certain that Henry made four errors, it is clear he did not commit less than four, but he might have committed five errors or more. Thus certainty can indicate lower bounds of an interval whose upper bounds are not defined, and certainty statements are consequently more ambiguous than estimates that do not mention certainty. At least-interpretations of certainty affect the interpretation of options in risky choice problems, where “200 lives will be saved” was deemed by a majority to mean exactly 200, while “it is certain that 200 will be saved”, could mean 200–600 lives. We also find that credibility is affected by type of certainty, with impersonal certainty (“it is certain”) perceived to be more accurate and persuasive than personal certainty (“I am certain”), especially in predictions of future events. Moreover, mentions of certainty can reveal that that a speaker's estimate is based on subjective judgments and guesswork rather than upon objective evidence. These findings have implications for communication. Estimates can appear more consensual when claims of certainty are omitted. To convey certainty it may be better not to mention that one is certain.