As a practitioner of psychology, Kahneman was baffled when he encountered the assumptions that underlie economics. The rational beings economists had designed their theories around were not the same beings Kahneman had been dealing with in his multitude of experiments!
Kahneman spends some time describing Bernoulli's Expected Utility Theory as a breakthrough that correctly predicts a number of behaviours. But it has a number of flaws, including the fact that it doesn't take a reference point (i.e. someone's current wealth) into account.
Kahneman's Prospect Theory is more complex as it requires a reference point, but it makes better predictions where Utility Theory fails. In addition, it incorporates the fact that humans appear to hate risk more than they like gains.
Finally, Kahneman discusses the ins and outs of the Endowment Effect.