Pessimism bias
Expecting outcomes to turn out worse than the odds actually warrant.
What it means
A systematic tendency to overestimate the likelihood of negative events and underestimate the likelihood of positive ones happening to oneself — the mirror image of optimism bias. It is associated with depressed and anxious mood, where the asymmetric belief-updating that normally favors good news is dampened or reversed, so bad news sticks and good news is discounted. Mild, situation-specific pessimism can be adaptive ('defensive pessimism'), lowering expectations to blunt disappointment and motivate preparation, but pervasive pessimism corrodes initiative, health behavior, and well-being. A long-debated wrinkle is 'depressive realism' — the claim that mildly depressed people sometimes judge their control and risks more accurately than cheerful optimists — though its scope and robustness remain contested. It matters because forecasts colored by chronic pessimism distort planning just as surely as rose-tinted ones.
Examples
An anxious student convinced they will fail an exam they are well prepared for, and discounting each piece of reassuring feedback.
A jobseeker with strong interviews behind them reads every silence as rejection, and treats the recruiter's warm follow-up as mere politeness rather than the good sign it is.
Someone certain their savings will never stretch to a deposit stops viewing flats altogether, waving away the mortgage adviser who has just shown them the numbers work.