We have seen in the previous chapter that in order to make effective decisions, people need to understand the risks and benefits associated with the different options that they face. They also need to understand the limits of their knowledge, as well as the limits of any 'expert' advice that they are given. This means that people not only need to be given reliable information about risks and benefits, but they also need to be able to understand the information and its limitations so that they can use it effectively. However, as we will see in the sections that follow, there is now a firm body of evidence showing that many people are subject to various cognitive difficulties when interpreting probabilistic information. Furthermore, we are all prone to a range of cognitive biases, and our interpretation of risk information is often influenced by the particular way in which it is presented to us. Finally, we will see that a number of emotional and affective factors will also play a role in how we interpret and respond to particular risk communications.
In order to choose effectively between different options, the decision maker has to make judgements about, and compare, the probability of different outcomes. There are various normative statistical methods for doing this which should produce the 'correct answer'. However, a large body of work, primarily associated with Amos Tversky and Daniel Kahneman, has shown that, in many everyday situations, people tend to use simpler 'short-cut heuristics' or 'rules of thumb' (see Tversky and Kahneman 1974 for an overview). Such heuristics frequently lead to the correct answer but are sometimes associated with systematic errors or biases. The two heuristics identified by Tversky and Kahneman that have received the most