This study identified a novel and robust reasoning error. Lay reasoners significantly deviate from the prediction of Bayesian inference by consistently underestimating the added probative value of corroborating testimonial reports. Most surprisingly, however, is that in certain contexts the sum of corroborating evidence is considered to be significantly less valuable than a single report. There is a selective devaluing of corroborating testimony when a highly reliable report is corroborated by less reliable, but credible, reports. This intuitive error is not explained by an inaccurate understanding of individual cues of reliability and number of reports, but specifically when it is required to integrate these both cues. Findings indicate the operation of alternative reasoning strategies, resulting in errors at individual and group level.

Testimony is a fundamental feature of human life: typically, wereceive testimonial evidence from others multiple times each day.Often, we have more than one source attesting to a particularclaim. This paper examines the way people integrate testimonialevidence from multiple sources. We find evidence that participantsdeviate substantially from the normative expectation. Instead,results seem indicative of the operation of simple, non-compensatory heuristics, at least some of the time.