Peak-End Rule

EXPERIENCE AND MEMORY

How can experienced utility be measured? How should we answer questions such as “How much pain did Helen suffer during the medical procedure?” or “How much enjoyment did she get from her 20 minutes on the beach?” The British economist Francis Edgeworth speculated about this topic in the nineteenth century and proposed the idea of a “hedonimeter,” an imaginary instrument analogous to the devices used in weather-recording stations, which would measure the level of pleasure or pain that an individual experiences at any moment.

Experienced utility would vary, much as daily temperature or barometric pressure do, and the results would be plotted as a function of time. The answer to the question of how much pain or pleasure Helen experienced during her medical procedure or vacation would be the “area under the curve.” Time plays a critical role in Edgeworth’s conception. If Helen stays on the beach for 40 minutes instead of 20, and her enjoyment remains as intense, then the total experienced utility of that episode doubles, just as doubling the number of injections makes a course of injections twice as bad. This was Edgeworth’s theory, and we now have a precise understanding of the conditions under which his theory holds—and, critically, when it does not.

The Peak-End Rule and Duration Neglect

The graphs in Figure 15 show profiles of the experiences of two patients undergoing a painful colonoscopy, drawn from a study that Don Redelmeier and I designed together. Redelmeier, a physician and researcher at the University of Toronto, carried it out in the early 1990s. This procedure is now routinely administered with an anesthetic as well as an amnesic drug, but these drugs were not as widespread when our data were collected. The patients were prompted every 60 seconds to indicate the level of pain they experienced at the moment. The data shown are on a scale where zero is “no pain at all” and 10 is “intolerable pain.” As you can see, the experience of each patient varied considerably during the procedure, which lasted 8 minutes for patient A and 24 minutes for patient B (the last reading of zero pain was recorded after the end of the procedure). Patient A’s procedure was shorter and included a period of intense pain near the end. Patient B’s procedure was longer but featured a gradual reduction in discomfort, ending far less painfully. A total of 154 patients participated in the experiment; the shortest procedure lasted 4 minutes, the longest 69 minutes.

Next, consider an easy question: Assuming that the two patients used the scale of pain similarly, which patient suffered more? No contest. There is general agreement that patient B had the worse time. Patient B spent at least as much time as patient A at any level of pain, and the “area under the curve” is clearly larger for B than for A. The key factor, of course, is that B’s procedure lasted much longer. I will call the measures based on reports of momentary pain hedonimeter totals.

When the procedure was over, all participants were asked to rate “the total amount of pain” they had experienced during the procedure. The wording was intended to encourage them to think of the integral of the pain they had reported, reproducing the hedonimeter totals. Surprisingly, the patients did nothing of the kind. The statistical analysis revealed two findings, which illustrate a pattern we have observed in other experiments. Together, they form the peak-end rule, a cognitive bias describing how people’s memories of past experiences are not based on the total sum or average of all moments but rather on how they felt at the most intense point and at the end.

• Peak-end rule: The global retrospective rating was well predicted by the average of the level of pain reported at the worst moment of the experience and at its end.
• Duration neglect: The duration of the procedure had no effect whatsoever on the ratings of total pain. The length of an experience has almost no impact on its remembered utility.

You can now apply these rules to the profiles of patients A and B. The worst rating (8 on the 10-point scale) was the same for both patients, but the last rating before the end of the procedure was 7 for patient A and only 1 for patient B. The peak-end average was therefore 7.5 for patient A and only 4.5 for patient B. As expected, patient A retained a much worse memory of the episode than patient B. It was the bad luck of patient A that the procedure ended at a bad moment, leaving him with an unpleasant memory. Patient B’s memory, shaped primarily by the more comfortable ending rather than the longer duration or total pain, reported a significantly less painful experience overall.

We now have an embarrassment of riches: two measures of experienced utility—the hedonimeter total and the retrospective assessment—that are systematically different. The hedonimeter totals are computed by an observer from an individual’s report of the experience of moments. We call these judgments duration-weighted, because the computation of the “area under the curve” assigns equal weights to all moments: two minutes of pain at level 9 is twice as bad as one minute at the same level of pain. However, the findings of this experiment and others show that the retrospective assessments are insensitive to duration and weight two singular moments, the peak and the end, much more than others. So which should matter? What should the physician do? The choice has implications for medical practice. We noted that:

• If the objective is to reduce patients’ memory of pain, lowering the peak intensity of pain could be more important than minimizing the duration of the procedure. By the same reasoning, gradual relief may be preferable to abrupt relief if patients retain a better memory when the pain at the end of the procedure is relatively mild.
• If the objective is to reduce the amount of pain actually experienced, conducting the procedure swiftly may be appropriate even if doing so increases the peak pain intensity and leaves patients with an awful memory.

Which of the two objectives did you find most compelling? I have not conducted a proper survey, but my impression is that a strong majority will come down in favor of reducing the memory of pain. I find it helpful to think of this dilemma as a conflict of interests between two selves (which do not correspond to the two familiar systems). The experiencing self is the one that answers the question: “Does it hurt now?” The remembering self is the one that answers the question: “How was it, on the whole?” Memories are all we get to keep from our experience of living, and the only perspective that we can adopt as we think about our lives is therefore that of the remembering self.

A comment I heard from a member of the audience after a lecture illustrates the difficulty of distinguishing memories from experiences. He told of listening raptly to a long symphony on a disc that was scratched near the end, producing a shocking sound, and he reported that the bad ending “ruined the whole experience.” But the experience was not actually ruined, only the memory of it. The experiencing self had had an experience that was almost entirely good, and the bad end could not undo it, because it had already happened. My questioner had assigned the entire episode a failing grade because it had ended very badly, but that grade effectively ignored 40 minutes of musical bliss. Does the actual experience count for nothing?

Confusing experience with the memory of it is a compelling cognitive illusion—and it is the substitution that makes us believe a past experience can be ruined. The experiencing self does not have a voice. The remembering self is sometimes wrong, but it is the one that keeps score and governs what we learn from living, and it is the one that makes decisions. What we learn from the past is to maximize the qualities of our future memories, not necessarily of our future experience. This is the tyranny of the remembering self.

Broader Implications

The peak-end rule and duration neglect have profound consequences across many domains. In medical ethics, they challenge the traditional focus on minimizing total suffering, suggesting that adding a phase of diminished discomfort at the end—even if it prolongs the procedure—can dramatically improve a patient’s memory of the event. Vacation planners, event organizers, and customer service managers can also leverage this bias: a holiday that ends on a high note or a purchase experience that concludes with a small, pleasant interaction will be remembered more fondly than one with greater total enjoyment but a mediocre ending. For personal decisions, being aware of this bias can help individuals make more rational choices about future experiences, basing them on predicted total utility rather than on the distorted memories shaped by peaks and ends.