This topic examines not just what goods and services consumers buy, but why they buy them. The standard neoclassical model, based on expected utility theory and indifference curve analysis, examines the ‘what’ question, but is too narrow in focus and involves numerous anomalies. To gain a better understanding of what people buy it is necessary to understand the psychology of consumers and examine the ‘why’ question. This approach reveals several important biases which cause the anomalies: biases in expectations, biases in estimating and maximizing utilities and biases in discounting. These biases are often a result of bounded rationality, social preferences and emotional or visceral influences. The field of behavioural economics has developed a body of theory, based on the concepts of prospect theory and mental accounting, which accounts for these biases and anomalies. The fundamental concepts here are the use of reference points and loss aversion. These and other behavioural factors are in turn based on evolutionary psychology. The process of natural selection has caused our brains to evolve not as utility maximising machines but as biological fitness maximising machines.