Underconfidence
Underconfidence is a systematic tendency to assign probabilities that are too close to 50% compared to reality. It appears when your 60% calls happen far more often than 60%.
Definition
Underconfidence means your probabilities are not extreme enough. You “hedge” toward 50% even when reality is more decisive.
How it appears in calibration
Underconfidence is a calibration issue. On a calibration diagram, realized frequencies are more extreme than your predictions.
Example: your 0.60 bucket resolves at 0.75, and your 0.40 bucket resolves at 0.25.
Impact on Brier score
Underconfidence usually hurts Brier score less than overconfidence on individual misses, but it can still reduce performance by leaving value on the table. If you are well calibrated but always near 50%, you may have low information content and low sharpness.
Common causes
• Fear of being “wrong” on record.
• Averaging conflicting signals without weighting reliability.
• Using 50% as a default even when evidence strongly shifts odds.
How to improve
• Start from a base rate and update with evidence, rather than starting at 50%.
• Review buckets where realized rates are consistently more extreme than your predictions.
• Evaluate skill with Brier skill score relative to a benchmark.
Related
Underconfidence is the opposite of overconfidence and interacts with sharpness and calibration.