The notion ‘utility’ has two different meanings. Jeremy Bentham argued that people are under the governance of two masters: pleasure and pain. They determine what we shall do and what we ought to do. Kahneman refers to this idea as ‘experienced utility’. When economists use the term, they mean ‘wantability’, which Kahneman refers to as ‘decision utility’. Expected utility theory concerns the rationality rules that should govern decision utilities.

Both concepts of utility can coincide: when people want what they will like and like what they chose. There are several possible discrepancies between the forms of utility. Imagine you have to receive a daily painful injection. The pain is the same every day. Will you attach the same value to reducing the number of future injections from 19-17 as from 5-3? You would be willing to pay more for the first option. The decision utility of avoiding two injections is higher in the first option, which is remarkable because the reduction of pain is the same. It shows that experienced utility can be measured (number of injections) and that experienced utility is the criterion by which decisions should be assessed.

Economist Edgeworth argued that experienced utility could be measured by using a ‘hedonimeter’: an imaginary instrument that measures the level of pain or pleasure that someone experience at one moment. Time is an important factor in his theory.

In a study of the experiences of two patients undergoing a painful medical procedure, the patients were asked to indicate how much pain they experienced every 60 seconds. 0: no pain. 10: intolerable pain. The experience lasted 8 minutes for patient Y and 24 minutes for patient Z. Which patient suffered more? You would go for patient Z, as his procedure lasted a lot longer. After the procedure, they were asked to rate the total amount of pain they had experienced. The two main findings were:

• Duration neglect: the duration of the procedure did not influence the ratings of total pain at all.

• Peak-end rule: the global retrospective rating was predicted by the level of pain at the end of the experience and by the average level reported at the worst moment.

Patient Y retained a much worse memory of the experience than patient Z. It was bad luck that his procedure ended at a painful moment, which left him with a distressing memory.

The two measures of experienced utility, the retrospective assessment and the hedonimeter, are different. Judgments based on the hedonimeter are duration-weighted: it assigns equal weights to all moments. The retrospective assessment is insensitive to duration and weights two singular moments (the end and the peak). Which one is best? For the medical practice, this is an important question. It depends:

• If the physician wants to reduce the memory of pain, minimizing the peak intensity of pain is better than lowering the duration of the procedure. Also, gradual relief is better than abrupt relief.

• If the physician wants reduce the actually experienced amount of pain, he should lower the duration of the procedure.

It is likely that most people will prefer reducing the memory of pain. The dilemma demonstrates a conflict of interests between two selves: the remembering self (How was it, overall?) and the experiencing self (do I feel pain right now?).

Imagine being at a concert that gets stopped near the end due to a fight and you hear someone say: “My whole experience is now ruined”. That is wrong: the memory of the experience was ruined, not the experience. The experiencing self had an almost entirely nice experience and the bad end could not undo that. Confusing experience with the memory of it is a common cognitive illusion. The remembering self is the one keeping score and making decisions. However, decisions and tastes can be shaped by memories that are wrong. The memory (System 1) represents the most intense moment of pleasure or pain (the peak) and the feelings when the experience was at its end. A memory that neglects the duration of experiences will not serve our preference for lasting pleasure and brief pain.