Comparing the responses of participants in reasoning experi-ments to the normative standard of Bayes Theorem has been a popular empirical approach for almost half a century. One longstanding finding is that peoples belief revision is conser-vative with respect to the normative prescriptions of Bayes Theorem, that is, beliefs are revised less than they should be. In this paper, we consider a novel explanation of conserva-tism, namely that participants do not perceive information provided to them in experiments as coming from a fully reli-able source. From the Bayesian perspective, less reliable evi-dence should lead to more conservative belief revision. Thus, there may be less of discrepancy between normative predic-tions and behavioural data than previously assumed.