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."^[1][2] 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.^[2] Empirical research, however, has consistently demonstrated systematic deviations from temporal monotonicity. Cognitive heuristics, notably the peak–end rule, often significantly influence judgments in ways not predicted by temporal monotonicity, revealing discrepancies between normative principles and actual psychological processes.^[2]

Definition and Principle

Temporal monotonicity is a normative rule in decision theory that describes how experiences ought to be evaluated over time. Specifically, it assumes that extending an episode by adding additional neutral or unpleasant moments cannot improve its overall subjective value.

As Kahneman and Frederick (2002) write,

"The most obvious rule is temporal monotonicity: there is a compelling intuition that adding an extra period of pain to an episode of discomfort can only make it worse overall." ^[1]

This principle plays a foundational role in normative models of rational evaluation, where judgments are expected to follow consistent, logical structures.^[3]

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^[2]

According to temporal monotonicity, the longer exposure—containing more total discomfort—should be evaluated more negatively. However, participants tended to prefer the longer trial because it ended less unpleasantly, illustrating how retrospective evaluations are influenced more by the ending than by total duration or intensity,^[4] 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.^[2] However, due to duration neglect, the actual length of an experience is often disregarded in retrospective evaluations.^[5] 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."^[1] In this view, the rule represents an ideal of consistent judgment, even if human behavior frequently departs from it.

See also

• Peak–end rule
• Duration neglect
• Heuristics in judgment and decision-making
• Affective forecasting
• Normative ethics
• Expected utility theory

Further reading

• Kahneman, Daniel. Thinking, Fast and Slow. Farrar, Straus and Giroux, 2011.
• Fredrickson, B. L., & Kahneman, D. (1993). "Duration neglect in retrospective evaluations of affective episodes". Journal of Personality and Social Psychology, 65(1), 45–55.
• Loewenstein, G., & Prelec, D. (1993). "Preferences for sequences of outcomes". Psychological Review, 100(1), 91–108.