Abstract: The Ratio bias occurs when people judge an unlikely event as less likely or more surprising when its probability is presented in the form of equivalent ratios of small than of larger numbers. This phenomenon is universal in people`s daily decision-making, especially in managerial and medical decisions. The paper introduces two psychological mechanisms of the ratio bias, including norm theory and cognitive-experiential self-theory. We then discuss the factors that affect the ratio bias, including metacognitive skills, numeracy, cognitive perspective, and task characters, ect. We outline several issues that deserve further investigations.

Key words: ratio bias, norm theory, cognitive-experiential self-theory