The peak-end rule is the psychological heuristic by which people judge an experience almost entirely based on how they felt at its most intense point (the peak) and at its end — rather than as an average of all moments.

Kahneman's Cold Water Experiment

Participants held their hand in painfully cold water for two conditions:

• Condition A: 60 seconds at 14°C
• Condition B: 60 seconds at 14°C, followed by 30 seconds at 15°C (still painful but slightly warmer)

Participants rated Condition B as less unpleasant and preferred to repeat it — even though it objectively involved more total pain. The warmer ending dominated memory.

Design Implications

• Customer experience: The last interaction determines brand memory disproportionately
• Healthcare: Colonoscopy discomfort studies showed a slower, gentler end reduced patient-reported pain despite added time
• Travel: End-of-trip experiences dominate vacation memories
• Presentations: What you end on is what people remember

Design Rule

Engineer your peaks and endings deliberately. Everything in the middle matters less than you think.

Reference:

Wikipedia: Peak-End Rule

https://en.wikipedia.org/wiki/Peak%E2%80%93end_rule