Temporal monotonicity

Temporal monotonicity is a normative principle in decision theory and psychology that postulates adding a period of neutral or unpleasant stimulus to an experience can only logically make the experience "worse overall." According to this rule, evaluations of experiences should remain consistent over time, preserving a logical relationship between duration, stimulus intensity, and the final subjective assessment.
Empirical Violations
The Peak-end rule
The Peak-end rule, a psychological heuristic, is a notable example of a violation of temporal monotonicity. For example, Kahneman et al. (1993) found that participants were more likely to wish to repeat an experience where they exposed their hands to 14°C water that increased to 15°C at the one-minute mark for a total exposure of 1 minute and 30 seconds, rather than a shorter exposure to 14°C water for 60 seconds further demonstrating how preferences for certain sequences can violate normative models like temporal monotonicity."
Duration Neglect
Similar to the peak-end rule, duration neglect plays an important role in cognitive appraisal that seems to ignore temporal monotonicity. According to temporal monotonicity, longer experiences (even if the additional duration is neutral or mildly unpleasant) should logically be judged as less pleasant overall. Instead, individuals tend to base their judgments on momentary peaks and endings, rather than the total time elapsed.
Despite consistent empirical violations, temporal monotonicity remains a foundational principle within normative models of rational evaluation. As Kahneman and Frederick (2002) note, these violations should be viewed as an "expendable flourish."<ref name=":0" /> In this view, the rule represents an ideal of consistent judgment, even if human behavior frequently departs from it.

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