Hindsight bias — "I knew it all along." After an outcome, people believe they predicted it.
1. Prevents learning — if you "knew," there's nothing to improve
2. Creates overconfidence in future predictions
3. Makes past decisions look obvious when they weren't
4. Leads to unfair self-criticism ("how could I miss that?")
Fix: Keep a decision journal. Record your reasoning before the outcome. Compare predictions to results. Humbling — and educational.
Five forecasts written down beforehand, then recalled after the outcome. On average the remembered forecast moved 22 points toward what happened. Scored honestly the forecasts have a Brier score of 0.35 (0 is perfect, 0.25 is a coin); the remembered ones score 0.14. Memory turned a below-coin forecaster into a good one.
- It is invisible from the inside. Only a record made before the outcome can show it, which is why a decision journal works and reflection alone does not.
- The opposite mistake exists too: judging a good decision by a bad outcome. Some bets are right and still lose.
- Biais & Weber (2009) found investment bankers with more hindsight bias earned less — it blocks learning, not just pride.
log = pd.DataFrame({
'event': ['rate cut', 'earnings beat', 'recession', 'oil above 100', 'index new high'],
'said': [0.30, 0.55, 0.20, 0.40, 0.60], # written down beforehand
'happened': [1, 0, 1, 0, 1],
'remembered': [0.60, 0.35, 0.45, 0.25, 0.80]}) # what you later recall having said
toward_outcome = np.where(log.happened == 1, log.remembered - log.said, log.said - log.remembered)
brier_said = ((log.said - log.happened) ** 2).mean() # lower is better
brier_remembered = ((log.remembered - log.happened) ** 2).mean()Assumes import numpy as np and import pandas as pd. Each snippet carries its own example numbers; the comments say which are made up.
- B. Fischhoff, “Hindsight ≠ Foresight: The Effect of Outcome Knowledge on Judgment under Uncertainty”, Journal of Experimental Psychology: Human Perception and Performance 1(3), 1975
- N. J. Roese & K. D. Vohs, “Hindsight Bias”, Perspectives on Psychological Science 7(5), 2012
- B. Biais & M. Weber, “Hindsight Bias, Risk Perception, and Investment Performance”, Management Science 55(6), 2009
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