The certainty effect is the psychological effect resulting from the reduction of probability from certain to probable (Tversky & Kahneman 1986).
Normally a reduction in the probability of winning a reward (e.g., a reduction from 80% to 20% in the chance of winning a reward) creates a psychological effect such as displeasure to individuals, which leads to the perception of loss from the original probability thus favoring a risk-averse decision.
However, the same reduction results in a larger psychological effect when it is done from certainty than from uncertainty.
Tversky & Kahneman (1986) illustrated the certainty effect by the following examples.
This demonstrates the typical risk-aversion phenomenon in prospect theory and framing effect because the expected value of option B ($45x0.8=$36) exceeds that of A by 20%.