  The Availability bias, manifested in the over-representation of extreme
eventualities in decision-making, is a well-known cognitive bias, and is
generally taken as evidence of human irrationality. In this work, we present
the first rational, metacognitive account of the Availability bias, formally
articulated at Marr's algorithmic level of analysis. Concretely, we present a
normative, metacognitive model of how a cognitive system should over-represent
extreme eventualities, depending on the amount of time available at its
disposal for decision-making. Our model also accounts for two well-known
framing effects in human decision-making under risk---the fourfold pattern of
risk preferences in outcome probability (Tversky & Kahneman, 1992) and in
outcome magnitude (Markovitz, 1952)---thereby providing the first
metacognitively-rational basis for those effects. Empirical evidence,
furthermore, confirms an important prediction of our model. Surprisingly, our
model is unimaginably robust with respect to its focal parameter. We discuss
the implications of our work for studies on human decision-making, and conclude
by presenting a counterintuitive prediction of our model, which, if confirmed,
would have intriguing implications for human decision-making under risk. To our
knowledge, our model is the first metacognitive, resource-rational process
model of cognitive biases in decision-making.
