The Peak-End Rule states that assessment of an experience is based on a combination of the peak emotional tone of the experience and how it ended.

In one study, participants were made to submerge their hand in cold water. One group held their hands in the water for a couple minutes. In the second group, participants left their hands in the water for an additional 30 seconds but during that time the temperature of the water was increased slightly. The second group reported a less unpleasant experience than the first even though they suffered for 30 seconds longer. Essentially, they remembered the whole thing as less cold because of how it ended. This shows that the ending of the experience has far greater influence on perception than the duration or actual amount of suffering.

This has some provocative applications. For instance, this was replicated in patients receiving colonoscopies. One group got a colonoscopy wherein the scope was left in for 3 extra minutes, but not moved, creating a sensation that was uncomfortable, but not painful. The other group underwent a typical colonoscopy. When asked to assess their experiences, patients who did the longer procedure rated their experience as less unpleasant than patients who did the typical procedure (even though they had three more minutes of discomfort).

When it comes to remembered happiness what matters is the peak emotional tone and how it ended.

REFERENCES

Kahneman, Daniel (2000). "Evaluation by moments, past and future" (PDF). In Kahneman, Daniel; Tversky, Amos (eds.). Choices, Values and Frames. Cambridge University Press. p. 693. ISBN 978-0521627498.

Redelmeier, Donald A; Kahneman, Daniel (1996). "Patients' memories of painful medical treatments: real-time and retrospective evaluations of two minimally invasive procedures". Pain. 66 (1): 3–8. doi:10.1016/0304-3959(96)02994-6